NSW Premier apologises for wearing Nazi uniform for his 21st birthday party

New South Wales Premier Dominic Perrottet has admitted he wore a Nazi costume to his 21st birthday party.

The state leader came clean to reporters in Sydney during a press conference.

He says received a phone call two days ago reminding him about the outfit choice and felt it was necessary to come forward and apologise.

"When I was 21, at my 21st fancy dress party, I wore a Nazi uniform," he said.

"I'm deeply ashamed of what I did. And I'm truly sorry for the hurt and the pain that it will cause people right across our state.

"Particularly members of the Jewish community, holocaust survivors, veterans and their families. I'm truly sorry for that terrible mistake."

He says the announcement wasn't designed to get in front of a story that was about to leak and he doesn't think there is a photo of him in the uniform that was about to be made public.

Premier Perrottet's excuse for wearing a Nazi costume was that he was 'naive' and didn't understand the hurt that could cause.

"It was stupid," Mr Perrottet said. "It was just a terrible mistake where I, at that age in my life, I just did not understand the gravity and the hurt of what that uniform means to people, not just in our state, but around the country and around the world."

This isn't the first time he realised the outfit was wrong.

His parents told him the day after his 21st birthday party that it was 'wrong and insensitive'.

Perrottet says he's lived with the secret in the nearly two decades since but believes the public has a right to know what happened from him.

"I have grappled with this, it’s something that has personally anguished me," he explained.

"I needed this truth about this terrible mistake that I made needed to be told by me not by someone else."

NSW Jewish Board of Deputies David Ossip revealed the Premier 'personally' reached out to offer his apologies over the matter.

"The Premier has been a staunch supporter and friend of the NSW Jewish community throughout his time in public life," they said.

"In particular, as treasurer, he ensured the Sydney Jewish Museum received funding to ensure that it could continue educating the community about the holocaust and the horrors of the Nazi era.

"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."