A $1 million reward is on offer as a fresh investigation was launched into the murder of one man and the suspected murder of another.
Detectives say they have "never given up" on solving the murder of Frederick Rosson, who was also known as Charles Johnson on the Gold Coast, and the suspected murder of Iain Stewart Hogg in Tweed Heads.
They believe the same people are responsible for killing both men in 2002, and say "it's never too late" for people to come forward.
Hogg is thought to have been shot dead on Monday 11 March 2002 in a carpark on Coral Street, Tweed Heads.
His distinctive red Toyota Celica was seen there and it's believed his body was dumped in the Tweed River.
It has never been found, and police are specifically appealing to the LGBTIQ+ community who may be able to help.
A few days later on Thursday 14 March 2002, Frederick Rosson is believed to have met two men at his home in Mt Nathan, Queensland to see a firearm he was selling.
Rosson was shot dead with his own gun.
A red Toyota Celica was seen at the property, believed to have been stolen after Hogg's murder.
That vehicle was abandoned at Palm Beach, Queensland, the following week.
Now NSW and Queensland governments have put up the cash as police announced they were stepping up the investigation.
NSW Minister for Police and Counter-terrorism, Yasmin Catley, said she hoped the reward would encourage those with information to come forward.
"Investigators have never given up on finding answers for the loved ones of Frederick and Iain, and today, the NSW Government reaffirms their ongoing support for the investigation," Catley said.
Tweed/Byron Police District Crime Manager, Acting Inspector Chris Elliott, said investigators believe that the same people killed both men.
"The families of these men deserve closure having dealt with the uncertainty of what happened over two decades ago," Elliott said.
Queensland Police Service Detective Acting Inspector Adam Bennett from Gold Coast Criminal Investigation Branch said police hoped the reward to lead to a breakthrough.
"It is never too late to come forward," Bennett said.
