Premier 'deeply ashamed' over Nazi costume

NSW Premier Dominic Perrottet has revealed he wore a Nazi uniform to his 21st birthday party, saying the costume choice was a grave mistake driven by youthful naivety.

"I'm deeply ashamed of what I did," he told reporters on Thursday.

"I'm truly sorry for the hurt and the pain this will cause right across our state and particularly to members of the Jewish community, Holocaust survivors, veterans and their families."

Repeatedly pausing as he answered journalists' questions, the 40-year-old Liberal leader denied he was anti-Semitic, adding the birthday party's theme was "uniforms".

Mr Perrottet said he only wore the rented costume once and did not recall any other offensive costumes being worn at the event.

He realised what he had done was wrong the next day when his parents, who were at the party, spoke to him about the costume.

"At that age in my life, I just did not understand the gravity of what that uniform meant," he said.

"It was just a naive thing to do."

The admission came after Transport Minister David Elliott on Tuesday approached the premier, according to Seven News.

"Political rivals knew about the costume - and everyone, including the premier's own staff, had heard the rumour - that someone was planning to use it against him," Mr Elliott told the broadcaster.

The incident occurred in 2003, a year after Mr Perrottet joined the NSW Liberal Party. In 2005, he was appointed president of the NSW Young Liberals.

The revelation comes after an anonymous Twitter account on January 5 posted claims of the existence of a "seriously damaging photo" relating to the NSW Liberal Party.

Mr Perrottet denied the admission was prompted by threats to release a photo, adding he wasn't aware an image existed.

Treasurer and deputy NSW Liberal leader Matt Kean said he supported the premier.

Mr Perrottet stands 10 weeks from his first election as premier, attempting to lift the coalition to a fourth successive victory at the state poll.

Before the self-inflicted damage, his government had already been weakened by the retirements of a dozen colleagues and the party's internal conflict over the selection of female candidates.

The NSW Jewish Board of Deputies said the premier had personally conveyed his "deep and sincere regret about his poor choice of costume as a young man" in a conversation shortly before Thursday's press conference.

"This incident, no matter how old, is a reminder of the need to continually educate all Australians - and particularly our youth - about the abhorrent nature of the Nazi regime and the evil perpetrated in service of the Nazi ideology," president David Ossip and chief executive Darren Bark said in a joint statement.

Nazi symbolism was not to be taken lightly and dressing as a Nazi was not a joke, which the premier had acknowledged, they said.

But they added the premier was a staunch supporter of the NSW Jewish community, securing funding for the Sydney Jewish Museum when treasurer.

Federal Liberal MP Julian Leeser, who is Jewish and whose electorate overlaps Mr Perrottet's state seat, said the premier he knew was "a world away from the arrogant, ignorant, heartless and mean-spirited actions of a university student."

However, cultural historian Jordana Silverstein said the incident spoke to a narrowness of the political vision of white Australia.

"The fact that he's only felt it appropriate to come out and reckon with this part of his past now - this is about face-saving, rather than a genuine accounting for what he has done," the University of Melbourne academic told AAP.

"It's always been considered offensive, but it's a matter of whose opinions have been listened to and respected."

Wearing swastikas, displaying Nazi memorabilia and waving Nazi flags is a crime in NSW, punishable with up to a year in prison and a fine of up to $11,000 after new laws passed the NSW parliament in August.