Prime Minister Malcolm Turnbull has reassured the families of the Port Arthur massacre victims they have not been forgotten.

About 500 people have attended a memorial service as rain fell at the historic site, which was the scene of Australia’s worst mass shooting 20 years ago.

Thirty-five people were gunned down by lone gunman Martin Bryant.

• Port Arthur: massacre haunts 20 years on
• ‘The guilt just ate at me’: Port Arthur survivors
• Waleed Aly slams Port Arthur filmmaker

In his keynote address Mr Turnbull said despite the passing of years and despite the healing, the sense of loss remained.

“We commemorate your resilience, just as we mourn the loss of many innocents, including our innocence,” he said.

There was emotion in his voice as he spoke of what might have been for the victims, and dreams that will never be fulfilled.

“The ripple in the pond from the loss of just one life is profound,” Mr Turnbull said.

“The evil that was committed here must never be allowed to overshadow the memory of each of those individual lives … of how they were loved and cherished before they were taken from us.

“You, the victims of cruel hatred have responded with love.”

He paid tribute to police, ambulance and hospital workers, “those who willingly run towards danger when most of us run from it”.

Mr Turnbull also paid tribute to gun control measures adopted by former prime minister John Howard and said the government would ensure agencies were equipped do deal with the threat of illegal firearms.

Wreaths laid near Broad Arrow Cafe site

Mourners gathered at the memorial pool to reflect on the tragedy. Photo: Getty

Wreaths were laid in the memory of the 35 victims by Mr Turnbull, Opposition Leader Bill Shorten and Tasmanian Premier Will Hodgman.

The wreaths were laid at the memorial pool near the remains of the Broad Arrow Cafe where 20 people died, and relatives gathered there shed tears.

The historic site’s management team only committed to mark the sombre anniversary in recent months at the urging of family and friends still grieving their loss.

Aware that many people feel such commemorations only keep a painful memory alive or even sustain the notoriety of the perpetrator, the organisers were determined to strike the most respectful tone.

Professor Sharon Sullivan from the Port Arthur Historic Site told the crowd that a decision had been made 10 years ago that the 10th anniversary would be the last one to commemorate the event.

But the message from some relatives of the victims had been consistent that their loved ones should not be forgotten.

A family stands at the memorial cross during a service marking 20 years since Australia’s worst mass shooting. Photo: Getty

Tasmania’s Governor Kate Warner said the event of 20 years ago added another layer to the former convict site, which had endured so much.

She said gun law reform was a positive and long lasting outcome, and the massacre was a reminder to remain vigilant.

She said the toll on the community had been “punishing” but that they had pulled together to reclaim a place that had become one the country’s greatest tourist attractions.

She acknowledged that for some, the pain would never end.

John Howard at the commemoration, with Julie Bishop and Bill Shorten. Photo: ABC

Australian opera singer Amelia Farrugia, who was at Port Arthur on the day and missed the gunman’s attack in the Broad Arrow Cafe by minutes, performed Andrew Lloyd Webber’s Pie Jesu.

A minute’s silence was held at the end of the ceremony, marking the time the massacre began.

After a morning of gentle rain, the sun broke through as the service wound up.

The day was also marked with ceremonies in Melbourne and at St David’s Cathedral in Hobart where bells tolled 35 times and 35 candles were lit to remember the lives lost.