The first time William Yang went to a dance party in Sydney in the 1970s, he was amazed to see so many people openly celebrating their sexuality.

"I was astonished to be in a room with over a hundred good looking guys," he says.

While there were underground queer dance parties before that time, this was the first time Yang saw people "that weren't so furtive, people were quite openly gay".

Yang, a social documentary photographer who's now in his 70s, has had backstage access to Sydney's queer dance parties for over four decades, documenting them from their beginning in the 1970s (the first Mardi Gras was in 1978) until today.

At the very beginning, he says, dance parties were a big part of the culture of gay liberation.

"People were celebrating the breaking of a thousand years of oppression."

"I was a photographer, which means that I was a voyeur," Yang says. ( ABC Arts: Teresa Tan )

Yang's work, which at times has explored his own queer Chinese-Australian identity, has been exhibited at major institutions including Sydney's Museum of Contemporary Art, the National Gallery of Australia and Brisbane's Gallery of Modern Art.

Since 1989 he's been presenting his photos as projections alongside spoken-word performances.

With the help of DJs Stereogamus, this month Yang is performing PARTY (verb) — telling the story of Sydney's queer underground dance parties, at the Sydney Opera House.

His party photos offer a glimpse of the heady nights (and days) of a scene that's provided an escape in the darkest of times.

Kandy Johnson performing at Ken's Karate Klub in the late 1970s. ( Supplied: William Yang )

As a freelance photographer, Yang took images that made their way into the pages of many Sydney magazines and newspapers.

One of Yang's early subjects was Ken's Karate Klub, a party in a sauna hosted by the drag queen Kandy Johnson.

"I was a photographer, which means that I was a voyeur and I was always a step back from what was happening because I was just focusing on the visuals. I knew that there was music there, but I hardly heard it," Yang says.

"I could wander around the backrooms and take photos ... people would let you do something like that because this was part of the gay liberation process: you don't have to be ashamed anymore."

Yang remembers Sydney's first warehouse party as a watershed event. ( Supplied: William Yang )

Yang says the first warehouse party in Sydney was in 1981.

"My friends were very excited about this. A new type of music was introduced called House music, created by Frankie Knuckles [a Chicago-based DJ]," Yang says.

"[This was] a watershed event — big dance parties became part of the culture ... The scale of some of the dance parties was huge."

PARTY (verb), which was commissioned by Performance Space and performed for the first time last year at Day for Night, is the first time he's shown these party photos from 1986-1992.

Jac Vidgen, pictured here in 1988, was the driving force behind the famous RAT parties. ( Supplied: William Yang )

It includes photos of Sydney's famous RAT (Recreational Arts Team) parties and Sleaze Balls.

"What made the RAT parties so appealing was that the RAT team [led by Jac Vidgen] tried hard to make it more than a dance party with lights and music — they had decorations, one had a food installation that you ate through the night," Yang says.

Important queer artists both attended and produced works for these parties, including Peter Tully, David McDiarmid and Billy Yip.

"People would take partying seriously; it was a cultural, fashion statement with performance [elements]," Yang says.

Yang believes that RAT parties — like this one, in 1986 — were victims of their own success. ( Supplied: William Yang )

"The RAT parties became more and more popular and then Jac [Vidgen] tried to broaden their appeal."

Later RAT parties featured BMX bike riders and wrestlers.

"But then they became less queer ... a lot of straight people came to these parties in the end," Yang says.

"That was [what] probably contributed a bit to the decline of the dance parties, there were too many bogans coming overall."

The Sleaze Ball, seen here in 1991, was another event photographed by Yang. ( Supplied: William Yang )

Yang says that during the dance party era of the late 1980s to early 1990s, "a kind of craze swept over Sydney".

"It was a delirium and there was a dance party every week. Thousands of people would go to a dance party and sometimes on the long weekends there'd be three or four," he says.

"I've looked back at the time and it coincided exactly with the AIDS era, where the community was in turmoil.

"The dance party was, I think, a perfect escape from that grim reality ... you could go and party and forget things.

"The fever died down after that [era] — like a craze or a fashion — but they would still have parties ... just nothing to that scale.

"There are actually quite a lot of little pockets of smaller dance parties now, because people still love to party."

Yang is still photographing parties, but the events are on a different scale. ( Supplied: William Yang )

These days, Yang documents more private events, and parties like the ones held by Unharm, an organisation that advocates for safe drug use.

"[But] there's not the same sense of community now that there was back in the 80s and the 90s," he reflects.

"Oxford Street doesn't exist anymore, and there's a different socialisation happening with the internet. The need to meet up is different."

William Yang will be performing PARTY (verb) at the Sydney Opera House, for Festival UnWrapped on May 10 and 11.