NSW Treasurer Matt Kean says “horrible cowards” forced Dominic Perrottet’s confession about wearing a Nazi costume to his 21st birthday party, and challenged them to come forward.

The premier remains under pressure after admitting to wearing a Nazi uniform to the event in 2003, at a press conference last week.

His public apology came after a phone call with Transport Minister David Elliott last Tuesday, who advised Mr Perrottet of rumours being spread about the party.

Today, the premier and treasurer were again asked about the saga at a media call marking the start of the state government’s new land tax scheme.

Last week, it was revealed Transport Minister David Elliott first raised the issue of the costume with Mr Perrottet, but its unclear who else knew about the uniform, and whether they were trying to trash the premier’s reputation.

“Well I don’t know who it is,” Mr Kean said, when asked if there were people within the Coalition conspiring against Mr Perrottet.

“But these horrible cowards should come out from the swamp that they’re living in and put their name to these horrible smears.

“These people should have no place in Australian public life. They want to play their silly games. Well, we’re not going to indulge that.”

It remains unclear if a photo of Mr Perrottet wearing the costume exists.

Mr Kean rejected the assertion that he was “potentially” calling members of his own government cowards, which did not present a united front ahead of March’s state election.

“What I said was whoever is doing these things should put their name to it, should come out, rather than hiding anonymously behind a keyboard pushing smear,” he said.

“That’s not acceptable.

“Put your name to it. Have some courage. Stop being a coward.”