The colloquial Australian-English phrase six o’clock swill, also 6 o’clock swill, denotes the customary bout of hasty drinking in public houses at the end of the working day, occasioned by the former six-o’clock-closing regulations.
—Cf. also ‘beer o’clock’: 5 p.m. as the end of the working day.

The earliest occurrence of the phrase that I have found is, as six o’clock swilling, from the Daily Telegraph and Daily News (Sydney, New South Wales) of Sunday 24^th September 1944:

Pubs
“Will someone take me to a pub?” wrote G. K. Chesterton. ¹
But what sort of pub?
Chesterton loved the mellowness and the mateship, the comfort and the courtesy, of the traditional English tavern.
He wouldn’t have found any of these things in the crowded, uncouth bars of Sydney today.
He would certainly have agreed with Professor Wilkinson’s lament that “in Australia, the Government prevents the pubs being used for the most sensible hours of the day.”
When the excuse of wartime necessity disappears, we should demand a new and rational approach to drinking.
We should revise our licensing hours to eliminate the bestial six o’clock swilling, and re-educate the public to leisurely drinking in cafes and lounges.

¹ Gilbert Keith Chesterton (1874–1936) was an English essayist, novelist and critic.

The phrase then occurs, as six o’clock swill and 6 o’clock swill, in newspaper accounts of a statement made by N. H. Connolly, President of the United Licensed Victuallers’ Association—accounts published on Wednesday 4^th October 1944; for example:

– In The Telegraph (Brisbane, Queensland) of Wednesday 4^th October 1944:

DRINK REFORM MOVE BY TRADE

Sydney: A more rational approach to drinking with the elimination of the “six o’clock swill,” through stagger hours, was advocated today by the United Licensed Victuallers’ Association president (Mr N. H. Connolly).
A Federal committee of the association had been set up in all States to examine the present drinking system, and its recommendations would be placed before the various Governments, he said.
“We don’t want an extension of present hours, but wish to give the public service when they mostly need it—in the hours when they relax and appreciate a drink,” Mr Conolly said.
Pre-war drinking hours, from 6 am to 6 pm, could be staggered, so that people could go home after work, have their evening meal and return for a refresher in genial company.
Even those opposed to drinking could find nothing to say in favour of the present mad 6 o’clock rush, he said. Under a better system, but still using the same number of hours, hotels could close for one or two hours late in the afternoon, and reopen at night.
At present men who knocked off work between 4 pm and 6 pm waited until the beer ran out before going home. After drinking so much many must be unable to enjoy an evening meal.
“I would like to see establishment of leisure drinking in hotels, where people could meet in a club atmosphere, similar to that existing In British inns,” said Mr Connolly. “Till we reform present drinking hours there is little chance of controlling the wild rushes at peak drinking hours.”
The secretary of the Australian Wine Producers’ Association (Mr C. S. Panton) said the constant aim of the organisation was to establish sane drinking hours, where people could partake of good Australian wines.
“We want to see people drinking under the best conditions, as with meals or along the lines of the Continental system, in the open air,” he added.

– In The Singleton Argus (Singleton, New South Wales):

DRINKING HOURS IN HOTELS
ATTEMPT TO AVOID “6 O’CLOCK SWILL”

Announcing that the United Licensed Victuallers Association was advocating changed drinking hours to avoid “the 6 o’clock swill,” the president, Mr. N. H. Connolly, said to-day that a Federal committee of the U.L.V.A. had been established in all States, and its recommendations would be submitted to each State Government.
The Association did not wish to extend the present hours, but wanted to give the public service when they mostly needed it—in hours when they could relax and appreciate a drink, said Mr. Connolly. He added that several suggestions had been made to improve services at country hotels. One was for the establishment of cafes as part of hotels, so that travellers could get a meal after hotel dining-rooms had closed.

The phrase six o’clock swill occurs, together with another phrase, 6 p.m. two-fisted drinking, in the following from the Daily News (Perth, Western Australia) of Saturday 7^th October 1944:

‘Civilised’ Drinking Planned

Sydney, Sat—Plans of the Federal Committee of the Licensed Victuallers Association, formation of which was announced in Sydney yesterday, were as yet nebulous, said Federal Secretary Corridon of the association.
“Our aim is to formulate plans for postwar reconstruction of the trade in order to extend a better service to the public,” he said.
Six o’clock closing would be a major question.
Former Tasmanian Premier the late Mr A. G. Ogilvie spoke of “6 p.m. two-fisted drinking,” said Mr Corridon.
“Any proposal which would enable consumption of liquor in a civilised manner would be preferable to the six o’clock swill.”

Published in The Herald (Melbourne, Victoria) on Tuesday 10^th October 1944, the following is about the reaction of J. J. Ryder, Secretary of the Rational Liquor Control League, to the report of the Royal Commission on Licensing Laws:

Declaring that the report did not give any guidance for cleaning up the liquor trade Mr Ryder said recommendations for increased penalties for sly-grog selling fell far short of what was needed. The League had expected sound proposals for stamping out black markets and the sly-grog trade, but members were disgusted at the apathetic tone of the report.
The big question of the 6 o’clock ‘swill’ had been practically bypassed as had many other evils which needed remedying so urgently.

The author of the following letter to the Editor, published in The Age (Melbourne, Victoria) of Tuesday 18^th February 1947, used the phrase ten o’clock swill:

Drinking Hours
Now that the most “free and easy” State in the Commonwealth has declared so emphatically for the retention of the six o’clock closing of hotel bars, perhaps the last will be heard of the cheap taunt that six o’clock closing has been maintained merely at the behest of a comparatively small section of “wowsers.” ²
The fact is that many thousands of moderate drinkers who would not vote for prohibition for varying reasons believe in strict control of the liquor traffic and would favor even stricter control than is in evidence at present.
As for the so-called “six o’clock swill,” this is not so harmful as a “ten o’clock swill” would be. Victoria’s earlier experience of even later “swills” provided evidence of that.—E. SMITH (Caulfield).

² The noun wowser, also Wowser, designates a Puritanical enthusiast or fanatic, especially a determined or fanatical opponent of intoxicating drink.

A variant of the phrase occurs in the following letter to the Editor, published in The Sydney Morning Herald (Sydney, New South Wales) of Wednesday 21^st September 1949:

Liquor On Trains
Sir,—Perhaps the Commissioner for Railways, Mr. Garside, is right in refusing to allow liquor on his new air-conditioned trains.
In Europe, civilised drinking is the custom of the people, so this controversy would not have arisen. Liquor on European trains is as automatic as the wheels on which they run. In New South Wales we are not educated up to these refinements. We are accustomed to the six o’clock hog swill habit. Drink as much as you can whilst the going is good.
A de luxe train in Europe on which only milk and lemonade were served with meals would be an insult to the passengers.
Pymble. E. E. SHELDON.