NSW Premier Dominic Perrottet has apologised for wearing a Nazi costume to his 21st birthday party, saying it was a grave mistake.

At an extraordinary press conference, Mr Perrottet said he was telling “this truth” after a colleague alerted him to knowing of the costume.

Mr Perrottet said he had grappled with that poor decision “for my life” and had thought about speaking of it in the past.

“When I was 21, at my 21st fancy dress party, I wore a Nazi uniform,” he told reporters on Thursday.

“I’m deeply ashamed of what I did and 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.”

The 40-year-old said the admission was prompted by a colleague coming to him in recent days.

“When it was raised with me — this difficult truth of a grave and terrible mistake that I made at my 21st birthday party — to be told by someone else, I felt it was very important that it came from me,” he said.

Mr Perrottet apologised several times publicly to the Jewish community for any distress and hurt.

He said his choice of outfit decades ago “demonstrated how naive I was”.

“I didn’t understand the significance of that decision,” he said.

“The hurt and the pain of what that uniform represents, particularly to the millions of Jewish people who died in the Holocaust and the impact on them.”

Journalists asked why Mr Perrottet had chosen now to speak, with a state election to be held in March.

He replied again that he needed to tell his truth and had thought about raising it at other times “on life’s journey”.

Repeatedly pausing as he answered journalists’ questions, the Liberal leader denied he was anti-Semitic.

“At that age in my life, I just did not understand the gravity of what uniform meant,” Mr Perrottet said.

“It was just a naive thing to do — that was a terrible mistake from a 21-year-old who just had no depth or appreciation.”

His parents, who were at the party, raised the issue of the costume with him the next day, he said.

The father of seven has been premier since October 2021 and is 72 days from going to the polls for the first time as leader.