In Australian English, the noun dunny denotes a toilet, especially an outside toilet, and the slang phrase to bang like a dunny door, also to bang like a shithouse door, is used of an exceptional sexual partner.

This phrase plays on two meanings of the verb bang: to make a loud noise and to have sexual intercourse.

The phrase to bang like a dunny door occurs, for example, in Rocktober 2000 puts Seventies in the shade, by Bernard Zuel, published in The Sydney Morning Herald (Sydney, New South Wales) of Saturday 14^th October 2000:

In the ancient times of denim and AM radio, disco and a real Aussie dollar, the 1970s, the biggest rock station in the city used to call this month Rocktober.
But 2Sm didn’t know the half of it. The next two weeks will see more music action in this city than an average year in the days of Hon Nick Jones and Ian Macrae.
[…]
Tonight sees a man who may have been inspired by that quintessential ’70s figure Mrs Slocombe to don thick and badly applied make-up, Robert Smith of The Cure, take to the Entertainment Centre stage for the first of two concerts at the same time that Australia’s biggest band, Powderfinger, begins a four-night run at the Enmore Theatre.
But come Tuesday, they may all be outshone by the most lusted-after latin [sic] male since CHiPs’s Erik Estrada, Ricky Martin. On the back of a single clearly inspired by an Australian term last openly used in the ’70s, She Bangs (like a dunny door?), Martin’s bare-chested Latin pop will set more than make-up running at the Ent Cent.

The earliest occurrences that I have found of the phrase to bang like a dunny door, also to bang like a shithouse door, are as follows, in chronological order—the phrase was popularised, if not coined, by Barry Humphries [cf. note ¹]:

1-: From Bazza arrives on the silver screen, by ‘Batman’, published in The Bulletin (Sydney, New South Wales) of Saturday 19^th August 1972:

At last we have a film […] that shows off the genuine Australian in a well-rounded, exhilarating light—“The Adventures of Barry McKenzie.” ¹
As you know Bazza McKenzie is the creation of Barry Humphries […].
[…]
Barry Crocker as Bazza is beautiful. He shows off remarkable cultural achievements, like being able to open a tube of the chilled article with one hand and give himself a swift amber transfusion with the speed of light. He wears the big hat, the double-breasted R. G. Menzies ² suit and he is utterly unlike any Australian in Earls Court today. He is the mythical Australian seen through the eyes of The Sunday Times.
His language too has a curious Frank Hardy ³ flavor, containing sufficient ingenuity to take over the world. Some examples: “She bangs like a shithouse door in a gale,” “I’m as dry as a nun’s nasty,” “I’m as dry as a dead dingo’s donger,” or “I hope your chooks turn to emus and kick down your dunny.” And when an arrogant Pom asks Bazza about his convict origins he disarmingly replies: “Go and dip your left eye in cocky shit.”

¹ The Adventures of Barry McKenzie (1972) is an Australian comedy film directed by the Australian film director Bruce Beresford (born 1940), written by Bruce Beresford and the Australian comedian Barry Humphries (born 1934), and starring the Australian comedian Barry Crocker (born 1935). This film tells the story of an Australian yobbo on his travels to the United Kingdom. Barry ‘Bazza’ McKenzie was originally a character created by Barry Humphries for a cartoon strip in the British satirical magazine Private Eye. (Barry Humphries also created and interpreted Dame Edna Everage, a fictional character satirising the average Australian housewife.)
² This refers to the Australian politician Robert Gordon Menzies (1894-1978).
³ This refers to the Australian author Frank Hardy (1917-1994).

2-: From Bazza brings it up, the account of the official Australian world premiere of The Adventures of Barry McKenzie—account by Sandra Hall, published in The Bulletin (Sydney, New South Wales) of Saturday 21^st October 1972:

Barry Crocker plays him [i.e., Barry McKenzie] as a walking accident. The deadpan feeling of the comic strip has become all vulnerable, big-eyed and ingenuous. “If it was raining sheilas,” he says in the producer, Phillip Adams’ favorite line, “I’d be washed down the drain with a poofter.” And it’s because he never will realise his ambition to find a jam tart who bangs like a dunny door that the well-prepared (or warned) opening night audience loved him.

3-: From the review of A Dictionary of Australian Colloquialisms (Sydney: Sydney University Press, 1978), by Gerald Alfred Wilkes (1927-2020)—review by the Australian author and historian Geoffrey Dutton (1922-1998), published in The Bulletin (Sydney, New South Wales) of Tuesday 18^th April 1978:

Professor Wilkes modestly writes in his introduction: “All dictionaries are tentative, and a colloquial dictionary is most tentative of all. Comments and further information from readers will be welcomed…” Well, here goes. I couldn’t find a mention of “humdinger”; “trot” in the sense of “give you the trots,” diarrhoea; “piker” in the sense of one not fronting up to his obligations; “boob” for jail, and “burgoo’ for jail porridge. And also, perhaps, “she bangs like a shithouse door,” which to my horror I once discovered had been taught by my children to the Russian poet Yevgeny Yevtushenko ⁴, who had asked them for useful examples of Australian idiom.

⁴ This refers to the Russian poet Yevgeny Yevtushenko (1933-2017).

4-: From Lily on the Dustbin: Slang of Australian Women and Families (Penguin Books Australia Ltd., 1982), by the Australian author Nancy Keesing (1923-1993):

The emphasis in several collections of our slang makes it unequivocally plain that Australians find endless humour in coarse ocker expressions, but the other side of coinages like ‘sink the sausage’ and ‘bangs like a dunny door in a gale’ is spurious and pays out despair and disaster to many women and children.