The family of Scott Johnson hope a new $1m reward will help find the truth about his death and bring his killers to justice, 30 years after he died in what has been found to be a gay hate crime.

Johnson, an American mathematician who was based in Sydney, was discovered at the base of a cliff below a gay beat near Manly’s North Head in December 1988.

His death was initially ruled to be a suicide but coroner Michael Barnes last year found the 27-year-old fell from the cliff “as a result of actual or threatened violence” by people who attacked him over his sexuality.

The coroner, who criticised the initial police investigation as inadequate, found it likely more than one person set upon the “young and strong and fit” Johnson while he was at the known gay beat. “Homicide is more likely than either of the other two scenarios – accident or suicide,” he said.

On Sunday, police were joined by Johnson’s brother, Steve, to announce a $1m reward for information leading to the conviction of those responsible for the death. It comes after the case was referred to a specialist team that is conducting fresh inquiries.

Speaking on the anniversary of his brother’s death, Steve Johnson told reporters it was a solemn day but his family was extremely appreciative that the police were taking action.

“After 30 years I can finally say: Scott, the police are keen to find your killers,” Johnson said.

“The police are the ones that can help you tell us what happened that day. We’re with you now, we’re going to find out what happened, we may finally get answers and justice for you.”

The New South Wales police commissioner, Mick Fuller, appealed for anyone with information to come forward.

“It’s horrendous to believe that these people are still out in our community – existing, living, free, while Scott ... has died a terrible death,” he said.

“If you’re out there, if you know something, if you know who committed this crime, if you know who was at the scene at the time, there is a million dollars on the table to get to the bottom of the truth.”