The Australian-English expression horror stretch denotes a very unpleasant experience, an ordeal.

This expression originally designated a particularly rough stretch of road on the 6,500-mile round-Australia Redex Reliability Trial of August-September 1953, organised by the Australian Sporting Car Club, and sponsored by Redex, a company providing fuel additives for motor cars.

The following, from The Advertiser (Adelaide, South Australia) of Monday 31^st August 1953, explained what the Redex Reliability Trial of August-September 1953 consisted in:

Big Car Trial Starts
250,000 See Start Of Trial

SYDNEY, Aug. 30.
More than 250,000 people packed city, suburban and outer district roads today to give a rousing, noisy send-off to the 186 competitors in the 6,500-mile round-Australia Redex Reliability Trial.
[…]
Before the surviving cars return to Sydney at the end of 14 days, they will have travelled through NSW, Queensland, NT, SA, Victoria and back into southern NSW, and have been driven over some of the toughest roads in the Commonwealth.

The earliest occurrence of the expression horror stretch that I have found is from The Sydney Morning Herald (Sydney, New South Wales) of Wednesday 9^th September 1953:

MOST CARS ON TIME
137 At Alice Springs

ALICE SPRINGS, Tuesday.—With 4,200 of the 6,500 miles behind them, 137 cars in the Redex reliability trial had checked in here from Darwin by 11 o’clock to-night.
All the teams are weary, but only one car so far has failed to come the 978 miles stretch from Darwin to Alice Springs in the specified time of 24 hours. The last car is expected at 2 a.m.

‘HORROR’ STRETCH TO-DAY

The first car will leave Alice Springs on the “horror stretch”—638 miles to Kingoonya—at 8.30 a.m. to-morrow. ¹
[…]
A car which arrived from the south to-day reported that the road is in a shocking condition because of the cutting up it has received since recent heavy rains.
It is reported that two men have a four-wheel drive truck, a towrope, and a cashbox and are waiting at the Finke River crossing for Redex competitors.
This crossing is 100 miles south of Alice Springs.
The cars will travel for 300 miles through spinnifex ² and mulga ³ country before reaching the notorious gibber ⁴ tracks further south.
Facing the prospect to-morrow of the toughest stretch yet to come, from Alice Springs to Kingoonya, most teams elected to rest soon after their arrival here.

¹ Alice Springs is located in the Northern Territory, Kingoonya in South Australia.
² The noun spinifex denotes a grass with coarse spiny leaves and spiny flower heads which break off and are blown about like tumbleweed.
³ The noun mulga denotes a small acacia tree or shrub.
⁴ The noun gibber denotes a large stone or boulder.

The expression horror stretch then occurs in the following from The Sydney Morning Herald (Sydney, New South Wales) of Friday 11^th September 1953:

“HORROR STRETCH” TOLL

A string of broken-down cars litters the 638 miles of dusty bush track—the “horror stretch”—between Alice Springs and Kingoonya.

In the following from the Daily Telegraph (Sydney, New South Wales) of Friday 11^th September 1953, the expression horror stretch designates an elimination section that the Australian Sporting Car Club introduced on the last stage—Melbourne-Sydney—of the round-Australia Redex Reliability Trial of August-September 1953:

DECIDING SECTION OF TRIAL

Australian Sporting Car Club officials decided in Sydney last night to introduce an elimination section on the Melbourne-Sydney run.
The club president (Mr. D. O. Macfarlane) said he would not disclose where the test section was.
He said drivers would not be able to negotiate the section at all if crowds of onlookers hampered them.
Club officials will give Trial drivers details of the elimination section when they arrive in Melbourne.
Drivers will be expected to average 37 m.p.h. from when they leave Albury to the first control at the “horror stretch.”
They will be clocked into the stretch—of about 20 miles over almost virgin, hilly land, littered with boulders and tree stumps—by stopwatch.
They will be clocked out of the stretch on another stopwatch.
Both watches will be synchronised.
Drivers must average 41 miles an hour over the stretch to negotiate it without loss of points.
A senior club official said the cars would be timed to the nearest quarter of a minute over the section.
Mr. Macfarlane said he felt sure the elimination stretch would find the ultimate winner.

Very early, the phrase horror stretch came to be applied to any particularly rough stretch of road—these are two examples:

1-: From Atomic age shatters the peace of the jungle, by “a Special Correspondent who is now touring uranium deposits in the Northern Territory”, published in The Examiner (Launceston, Tasmania) of Monday 5^th October 1953:

Usually reticent mine management officers are enthusiastic about two new finds of uranium bearing ore close to the White mine.
Mount Fitch, reached after a “horror” stretch of eight miles through rugged mountainous country covered by inches of red dust and broken rock, is regarded as most promising.

2-: From The Murrumbidgee Irrigator (Leeton, New South Wales) of Tuesday 1^st December 1953:

At the meeting of the Leeton Shire Council on Thursday night the following correspondence was dealt with:
[…]
E. H. Hanby, Narrandera, complaining of the deplorable state of the gravel crossing on T.R. 80 road naming it the “horror stretch.”

The earliest figurative uses of the expression horror stretch that I have found are from The Daily Examiner (Grafton, New South Wales) of Saturday 17^th July 1954:

Horror Stretches

The roads of life are never smooth for long, as the Redex drivers are finding to their cost. We may apply this to the current affairs of to-day. The recent cyclone, for example, brought devastation to many districts and nearly unloosed another major flood on the Northern Rivers. It is some consolation to learn that Mr. Cahill ⁵ is spending money on a better system of flood warning. But it would cheer us more to be told that the Government was spending money on a system of flood control. Apparently the horror stretch of flood dangers is to be with us for many years yet.
In the political scene the going is still rough. The Petrov inquiry ⁶ reveals that some people have a strange idea of patriotism. Loyalty and service to one’s country seems to mean less than devotion to an alien ideology. We can only hope that the revelations of disloyalty and espionage will strengthen the love that the majority of us have for this mighty land of ours.
There is no room in our national life for divided allegiance. Indeed if we branded it by the old name of treason this horror-stretch trodden by a misguided few might well be avoided in the future. United we stand, divided we fall. An old adage certainly, but one that should be placarded on every horror stretch of subversive activity.
Even in the world of sport the old ideals seem to be falling. Tennis tantrums and football fracas may provide good copy for sensational columnists. Nevertheless they sadden the minds of many for whom sport was once an honourable relaxation. Let us hope that this horror-stretch in sport will never recur.
In economics certainly, the going seems better. The slump in U.S.A. as forecast by Colin Clark ⁷, is said to have been averted. Sir Arthur Fadden ⁸ has a pleasing Federal surplus. Some relief in taxation may be looked for. If inflation has really been halted and is not revived by the increase in margins there should be smoother financial roads ahead for us all. We certainly hope so. For the farmers on the low lands, and the fishermen especially have battled on far too long on the horror-stretches of financial stringency.
The restoration of a shipping service to the North Coast is also good news. Without ships our important rivers are wasted. They become mere avenues of futility and periodical danger. We could, of course, cite again the waste of the uncontrolled waters of the Clarence, did we not feel that readers may weary of our persistent advocacy. Yet we of the Clarence district are even more weary of the passing years in which so many promises are made and so little actually done. Surely one day we shall enter upon a smooth stretch of fuller rural development. The long Redex trial is a small thing compared with the long years of neglect patiently endured by the dwellers on this cinderella of the north.
The international scene is still full of pitfalls. Britain, America and France seem in accord over Indo-China. But the road surface changes almost from day to day. However Mr. Casey ⁹ expresses optimism, although Mr. Menzies ¹⁰ rightly warns us that vigilance is needed. Pakistan has endorsed our White Australia policy ¹¹ and says that the Asians have no thoughts of aggression. The Colombo Plan ¹² appears to be bearing good fruit. If our leaders can unite the great Powers in a common policy of co-operation and goodwill peace may be preserved and the horror-stretch of another world war be avoided.

⁵ John Joseph Cahill (1891-1959) was the 29^th Premier of New South Wales from 1952 to 1959.
⁶ Following the defection to Australia of Vladimir Petrov (1907-1991), Third Secretary of the Soviet embassy in Canberra, the Royal Commission on Espionage was established in April 1954 by the Australian government to inquire into and report on Soviet espionage in Australia.
⁷ This probably refers to the British and Australian economist Colin Clark (1905-1989).
⁸ Arthur Fadden (1894-1973) was then the Treasurer of Australia.
⁹ Richard Casey (1890-1976) was then the Minister for External Affairs.
¹⁰ Robert Menzies (1894-1978) served as Prime Minister of Australia from 1949 to 1966.
¹¹ The White Australia policy, formally Immigration Restriction Act of 1901, effectively stopped all non-European immigration into the country and contributed to the development of a racially insulated white society. This policy became less stringent after the Second World War; for example, under the Colombo Plan ¹², students from Asian countries were admitted to study at Australian universities.
¹² The Colombo Plan for Cooperative Economic and Social Development in Asia and the Pacific was conceived at the Commonwealth Conference on Foreign Affairs held in Colombo, Ceylon (now Sri Lanka) in January 1950 and was launched on Sunday 1^st July 1951. The original signatories were Australia, the United Kingdom, Canada, Ceylon, India, New Zealand and Pakistan.