Australia has announced a sweeping national firearm amnesty that will allow owners of illegal firearms to hand in weapons for the first time since the aftermath of the Port Arthur massacre in Tasmania in 1996.

The amnesty was one of the measures agreed as part of the revised 2017 National Firearms Agreement by the state and territory governments.

The amnesty has been precipitated by a steady increase in the number of unregistered firearms in Australia, which law enforcement and intelligence agencies now believe to number approximately 260,000.

The justice minister, Michael Keenan, said: “From three months until 30 September, anyone with an unwanted or unregistered firearm, or a firearm-related item such as ammunition can legally dispose of or register their firearm at approved drop-off points in each State and Territory.”

“My expectation is it will probably not be the case that we will have hardened criminals who have made a big effort to get a hold on illegal guns would necessarily hand them in. The purpose is to reduce the number of unregistered and illicit firearms in the community”.

Although Keenan announced the amnesty today, the amnesty cannot go ahead until state and territory governments pass laws putting it into effect.

The NSW government introduced its version of the amnesty laws in May, but it has not yet passed both houses.

Some other states and territories have also not yet passed legislation to give effect to the amnesty.

Fellow cabinet minister Christopher Pyne said if illegal guns weren’t in the system they couldn’t be used to kill the likes of Queensland police officer Brett Forte recently.

The Labor opposition party is backing the amnesty. “We would certainly encourage people to do the right thing and to hand them in,” frontbencher Anthony Albanese told Channel Nine.

The Port Arthur shooting in April 1996 ended with the deaths of 35 people at the popular tourist site in Tasmania. The gunman, Martin Bryant, was given 35 life sentences.