Malcolm Turnbull has used a memorial service for the victims of the Port Arthur massacre to promise to defend and possibly strengthen Australia's gun laws.

As about 500 people gathered in the rain to mark 20 years since Australia's worst mass murder by a lone gunman, an emotional Mr Turnbull praised the leadership of survivors who became "a powerful voice for change" in the months after the attack before national firearm laws were introduced.

It came as the laws are under review, and debate continues over the legality of the Adler A110 lever-arch shotgun.

"The national firearms agreement remains today as it was 20 years ago and we will, working together, ensure that the national firearms agreement remains a strong defence against gun crime," Mr Turnbull said.