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.

"Where we can do more, we will, including through legislation such as we have proposed to increase prison terms for importation of firearms and firearm parts."

Protected by umbrellas and blue plastic ponchos, dignitaries, survivors and victims' loved ones braved the cold and wet to remember the 35 people lost two decades ago.

For some victims' family members, it was the first time they had been back to the site since the shooting. Others stayed away, either still too raw to return or feeling they no longer needed to mourn in the glare of a nationally televised service.

The theme of the service was "we remember". It began with a lone piper playing next to the wooden cross in the historic site's memorial garden, next to the gutted remains of the Broad Arrow Cafe, where more than half the victims were killed. Mr Turnbull, former prime minister John Howard – the architect of Australia's gun laws - and Labor Leader Bill Shorten were among guests to lay wreaths at the foot of the cross.

Later, family members and friends of the dead hugged each other as they placed 35 floral arrangements in the memorial garden's reflective pool next to the engraved words of the late writer Margaret Scott: "May we who come to the garden cherish life for the sake of those who died."

The service was punctuated with song, scripture and moments of silence. Soprano Amelia Farrugia, who was at Port Arthur on the day of the massacre, sang Pie Jesu. Keith Moulton, who lost his daughter Nanette Mikac and granddaughters Alannah and Madeline, gave a reading.

Martin Bryant, the murderer now serving 35 consecutive life sentences at Risdon Prison outside Hobart, was not mentioned.

Mr Turnbull said despite the healing in the years since, the sense of loss still weighed heavy on the people directly affected, and the country. "We will never be the same," he said.

In her keynote address, Tasmanian Governor Kate Warner said the commemoration should also serve as a reminder.

"Remembering the Port Arthur massacre should remind us to be vigilant about weapons which can allow one man to kill so many people so quickly and efficiently," she said as the rain lifted.

"For many, the pain and anguish will never end."

The service also heard a message of resilience. The massacre had a devastating impact on the Port Arthur community, psychologically and economically, but hope had returned.

"The events of 20 years ago took a punishing toll on individuals and families, on businesses and on community confidence," Professor Warner said.

"But with enormous grit the community has pulled together. Port Arthur has reclaimed its place as one of Australia's greatest heritage attractions."