On the eve of the 40th anniversary of gay law reform in South Australia, Police Commissioner Grant Stevens says the force has an "excellent relationship" with the gay community.

Commissioner Stephens has used the approaching milestone to reflect on the event that precipitated the law change: the drowning of gay law lecturer Dr George Duncan in the River Torrens.

Dr Duncan's death on the night of May 10, 1972, remains an unsolved crime.

At the time, homosexuality was illegal in every Australian state and the Torrens was a well known gay beat.

Dr Duncan had been in Adelaide just six weeks when a group of men threw him into the river.

The 41-year-old could not swim and drowned.

Though never proven, suspicions were quickly raised about police involvement in the incident.

"I think that it's a reasonable presumption that an incident like this would have had an adverse impact on the relationship between the police and the gay community," Commissioner Stevens said.

At the time police were criticised, particularly by Dr Duncan's colleagues at Adelaide University Law school, for not pursuing the case more vigorously.

But Commissioner Stevens said the original investigations should not be criticised today.

"We are talking about something that happened 40 years ago," he said.

South Australian Police Commissioner Grant Stevens says the force has a great relationship with the gay community. ( ABC News: Martin Davies )

"From my understanding the original police investigation didn't produce sufficient evidence to make any findings, so the commissioner of the day organised for New Scotland Yard to send detectives over to do an independent investigation.

"I think that's as good as anybody could have expected at that time.

"Unfortunately, that investigation didn't result in any charges being laid either."

In 1975, Don Dunstan's Labor government became the first government in Australia to decriminalise male homosexuality.

The issue came to a head again in the late 1980s, when it was re-investigated after new information from a police officer.

Commissioner Stevens was a young officer in the force at the time.

"The outcome of that 1986 investigation was the charging of three [former] police officers with two going to trial, so it was not a good thing by any stretch of the imagination," he said.

The former officers were acquitted of Dr Duncan's manslaughter.

$200k reward for information on Duncan's death on offer

Commissioner Stevens said the case remained open on police files, with information about a $200,000 reward on the Crime Stoppers website.

University of Adelaide law lecturer George Duncan drowned in 1972 after being thrown into the River Torrens. ( SA Police )

He has chosen to speak about the Duncan incident because it was a catalyst for gay law reform in South Australia.

"It's clear that the death of Dr Duncan was a major factor in that reform being achieved, and SAPOL is a part of that history," he said.

"I think it's important to reflect on history.

"The other thing we want to achieve from this to demonstrate or provide evidence of the fact that SAPOL is a much more inclusive place.

"We have an excellent relationship with the gay and lesbian community and we want to build on that and I think part of building on that is recognising the past."

Commissioner Stevens said he served alongside openly gay officers.

Premier Jay Weatherill said it was encouraging the police had chosen to speak.

"I think it is a very significant moment," he said.

"There's no doubt this incident caused disquiet in the South Australian community and I think a measure of distrust between the gay community and the police.

"I think Grant Stevens has communicated to the gay community in South Australia that they can trust the police force."