More than 30,000 firearms are expected to have been handed in during Australia’s first national firearms amnesty since the national gun buyback prompted by the Port Arthur massacre in 1996.

Figures for most states and territories are yet to be finalised, but New South Wales police said they had received more than 20,000 guns between 1 July 1 and 30 September. Of those, more than 5,000 were surrendered in the past month.

Western Australian authorities announced on Monday that they had received 1,242 surrendered firearms during the amnesty period, including 186 shotguns, 860 rifles, 196 handguns and 65,628 rounds of ammunition.

The surrendered weapons included first world war rifles and handguns, a second world war sub-machine gun and luger pistols, as well as three guns from the 19th century.

Tasmanian police collected 1,924 firearms, comprising 754 shotguns, 1,071 rifles and 109 handguns.

The surrendered firearms included two SKS military-style semi-automatic rifles, a 150-year-old Belgian Lefaucheux 9mm pin-fire revolver plus a tin of original rounds, which had been used to guard the mail coach between Richmond and Hobart, and two guns that had been reported stolen since 1995: a .222 calibre rifle and a Norinco NZ759mm pistol.

While it is the first national firearms amnesty since the buyback scheme in 1996, under which more than 650,000 guns were collected and melted down, there have been a number of smaller, state-based amnesty periods.

WA police ran a statewide gun amnesty in 2013, which collected 1,281 firearms and 80,000 rounds of ammunition. South Australia has held five gun amnesties between 1997 and 2017, amassing a total of 13,252 firearms. In the most recent amnesty period, between December 2015 and June 2017, just before the national amnesty began, 5,472 firearms were surrendered.

A 150-year-old antique Belgian Lefaucheux 9mm pin-fire revolver and a tin of original rounds handed into Tasmanian police. Photograph: Ethan James/AAP

Queensland has held amnesties in 2004 and 2013, collecting and destroying 2,835 and 2,000 guns respectively. NSW reportedly collected 63,000 guns in two amnesties in 2001 and 2003 and a further 4,323 in 2009.

Tasmania and Victoria have had standing gun amnesties since 1997, meaning that a person can at any time surrender an unregistered firearm to a police station or gun dealer.

All firearms surrendered in the national amnesty will be destroyed, with the exception of a few antiques that have been passed on to state museums.

University of Sydney emeritus professor Simon Chapman, who has researched the decline in gun deaths since the national firearms agreement was put in place six weeks after the Port Arthur massacre, said he suspected many of the guns surrendered in 2017 should have been surrendered 20 years ago.

Chapman’s research found there had been no fatal mass shootings since the national firearms agreement was put in place and gun deaths had declined at an accelerated rate, from a 3% decline annually prior to 1996 to a 5% annual decline in the 20 years since.

“The guns which have been handed in in the most recent one are guns that people kept in defiance of the law since 1996,” he told Guardian Australia. “At the time I can recall going on to gun blogging sites such as they were at that stage, and people talking about hiding them and burying them and all that sort of stuff … there are obviously more out there.”

Chapman said the initial gun buyback and rolling gun amnesties were an important part of Australia’s gun control system because they reduced the likelihood that unregistered and unaccounted for guns would be available to people who had no proper purpose to have them.

Under Australia’s gun laws, semi-automatic or automatic firearms of the type suspected to have been used in the Las Vegas massacre cannot be legally owned and possession of one of these weapons, or any other unregistered firearm, carries a potential fine of $280,000 or 14 years imprisonment.

“Obviously there will always be some guns that come in illegally, but the world looks on to Australia’s gun laws with great envy,” Chapman said.

The NSW Greens MP David Shoebridge questioned the effectiveness of gun amnesties, which he said did not balance out the number of guns being imported, legally or illegally, into Australia. “NSW alone is on track to have a record 1 million registered firearms by 2020,” he said.

Shoebridge said the 2017 national firearms agreement, which resulted from the 20-year review of the Port Arthur laws, was focused on reducing red tape for gun owners.

Gun Control Australia will release an audit of the 2017 laws on Thursday.

“There is nothing in Australian law that stops a person from buying hundreds of guns and creating what are in effect private arsenals in suburban Sydney or Brisbane,” Shoebridge said. “This is a hole in the current laws that needs an urgent fix.”