"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 about the costume.

"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.

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.

Australian Associated Press