A former US state attorney-general who worked with the family of slain American mathematician Scott Johnson has slammed NSW police for "resisting" solving the case.

Mr Johnson was found dead at the bottom of cliffs in Sydney in 1988.

Martha Coakley, who served as Massachusetts attorney-general from 2007 to 2015, took on the prominent case after it was initially ruled a suicide.

NSW Police reopened an investigation in 2012 following pressure from Mr Johnson's family.

Ms Coakley on Friday told an inquiry that police put a lot of resources into resisting any change to the suicide finding or seeing if the case could be ruled a homicide.

"If that case had the appropriate resources from the beginning, instead of writing it off as a suicide, losing evidence, not talking to folks ... some of the progress could be made," she said.

Scott White pleaded guilty to Mr Johnson's manslaughter in the Supreme Court last week.

Ms Coakley was giving evidence to a royal-commission style inquiry into several NSW police strike forces that examined the cases of nearly 90 LGBTQI people found dead between 1970 and 2010.

The inquiry previously heard throwing gay men off cliffs near so-called "beats" was a common tactic among assailants.

Ms Coakley also took aim at Strike Force Parrabell, a much-heralded police review into 88 cases involving LGBTQI people who were found dead in NSW between 1976 and 2000.

It was established in 2015 to determine which of the deaths were potentially motivated by a gay-hate bias.

Its 2018 report found 27 cases were gay-hate bias crimes with eight confirmed and 19 suspected.

But Ms Coakley said the review was flawed as it didn't investigate still-open cases.

"To me, the report wanted to confirm for the public that we did this complete report, we did all this work for 18 months and here's the result," she said.

Commissioner John Sackar chided police lawyer Anders Mykkeltvedt for describing Parrabell in glowing terms for its "uniqueness" as a law enforcement exercise.

He said the inquiry heard evidence the taskforce was the direct result of negative publicity.

"This was not something dreamt up one morning just for the sake of it. It had a catalyst."

University of Tasmania policing expert Nicole Asquith also criticised Parrabell, saying it was ill-conceived from the onset as it failed to engage with the LGBTQI community.

Professor Asquith was among the academics who tendered to be an independent auditor of the review, but she wasn't chosen for the process.

Some of the other suspected cases that have been exhaustively pored over in the inquiry include the 1980s deaths of Ross Warren, John Russell and French national Giles Mattaini in Bondi.

The inquiry continues.