SYDNEY - The increased threat of terrorism and a flood of illegal firearms into Australia have prompted the government to offer its first national gun amnesty since a massacre at a former penal colony more than 20 years ago. In 1996, 35 people were killed by a lone gunman at Port Arthur in Tasmania. That tragedy prompted tough gun control measures.

It is estimated there are more than a quarter-million illegal guns in Australia, although government officials concede the exact number is impossible to gauge. A recent proliferation of illicit weapons and the potential impact on national security, as well as concerns about organized crime, have prompted the government in Canberra to introduce a three-month amnesty, starting July 1.

It will give Australians the chance to hand in their unwanted weapons without fear of prosecution. Anyone caught with an illegal weapon outside that period face fines of up to $212,000 or up to 14 years in prison.

Australian Justice Minister Michael Keenan says a deadly siege at a cafe in downtown Sydney in December 2014, the murder of a police accountant in Sydney a year later, and a fatal shootout this month in the Melbourne suburb of Brighton are evidence that an amnesty is needed.

“Well, it brings certainly into mind the importance of doing this. I mean, we know the Lindt Cafe siege an illegal firearm [was used]; the murder of Curtis Cheng, an illegal firearm [was used]. It is yet to be determined some of the aspects of Brighton, but obviously we would expect the same. So clearly the fact that we have a deteriorating national security environment, got an environment where there has been five terror attacks on our soil, and, sadly, in the vast majority of those cases, it has been an illegal firearm that has been used.”

A similar deal followed the Port Arthur shootings in the mid 1990s, which also restricted possession of automatic and semi-automatic firearms.

Thirty-five people died when a lone gunman, using a semi-automatic rifle, attacked a former penal colony at Port Arthur, a popular tourist destination on the island state of Tasmania. It was the worst mass shooting in Australia's history.

Officials say the tough gun control measures brought in after the Port Arthur shootings have prevented further mass killings.