Gun Control Australia says the estimated total number of firearms in the hands of civilians was now 3.2 million -- the same number as in 1996 (AFP Photo/David McNew)

Sydney (AFP) - Australia marked 20 years since its last mass shooting Thursday as campaigners warned its gun laws, which were severely tightened after the massacre, are being eroded.

Thirty-five people died at the historic Tasmanian colonial convict site of Port Arthur in 1996 as lone gunman Martin Bryant, armed with semi-automatic weapons, went on the rampage.

Now aged 48, he remains in jail after being sentenced to 35 life sentences without parole.

Then-prime minister John Howard reacted swiftly by pushing for tough gun laws, including bans on certain weapons, a minimum age, licences and secure storage.

Since then there have been no massacres on such a scale, in contrast to countries such as the United States where mass shootings are common, with guns being blamed for around 10,000 homicides each year.

Prime Minister Malcolm Turnbull, who was to attend a memorial service to mark the anniversary at the popular tourist site, insisted the laws would not be watered down.

"John Howard ensured that we have had, and have, the toughest gun control laws in the world and we are committed to ensuring they remain just that," he told reporters in Tasmania.

"We are committed to ensuring that our national firearms agreement becomes stronger and that Australians remain safe from gun crimes, like the shocking events at Port Arthur."

But Rebecca Peters, a campaigner to reform Australia's laws after Port Arthur and the 10 other mass shootings in the decade before it, said that while the changes introduced by Howard were "a very powerful step forward", they were slipping.

"Australia was recognised as having the best gun laws in the world," she told AFP. "Twenty years later, those gun laws are being eroded, we are slipping backwards."

Gun Control Australia agreed, saying the estimated total number of firearms in the hands of civilians was now 3.2 million -- the same number as in 1996.

The organisation's chair, Sam Lee, said while people must have a gun license to possess a firearm, some owners were accumulating many weapons, while unlicensed people were also able to shoot at gun clubs.

"We have seen a watering down of gun laws... as the gun lobby groups continue to prosper and pro-gun representatives to state and federal parliaments gain power," Lee said.

Research by Philip Alpers, a University of Sydney gun control expert and founding director of GunPolicy.org, backed the concern.

"The million guns destroyed after Port Arthur have been replaced with 1,026,000 new ones. And the surge only shows upward momentum," he said in a commentary for academic journal The Conversation.