The Australian Greens have called on the Turnbull government to do more to uphold the spirit of post-Port Arthur gun laws after it emerged firearms dealers are using a loophole to modify a controversial rapid-fire shotgun so it can shoot more rounds.

Plans to import the Adler A110 lever-action shotgun, which has a magazine capacity of seven rounds, were halted last July when it was banned by former prime minister Tony Abbott for six months, later extended until August 7 this year.

The importer, Robert Nioa, then sidestepped that decision by introducing a version of the gun that has a reduced five-round magazine, and so is not covered by the Abbott ban.

It has now emerged gun dealers are also sidestepping the ban on the seven-shot Adler by using a legal loophole that allows them to modify the weapon and boost its magazine capacity.

One gunsmith and dealer in South Australia, who openly markets the magazine extension tubes, is charging $250 to convert the Adler A110 to accommodate a 10-shot magazine, and one additional cartridge in the chamber.

The modification enables the weapon to fire 11 shots in as many seconds without the need for reloading.

Tasmanian Greens senator Nick McKim says the argument to ban the importation of lever-action shotguns with magazines had been "strengthened by unethical gunsmiths working around the current ban".

"Prime Minster Turnbull should uphold the spirit of John Howard's gun control reforms and ban the importation of lever-action shotguns with magazines," Senator McKim said.

"This kind of modification is clearly designed to undermine and subvert the existing ban on seven-shot lever-action shotguns."

"There is no legitimate argument in favour of seven-shot magazines for shotguns, let alone 10-shot magazines."

While the Adler A110 five-shot - available under the least restrictive firearms category - can be converted to a seven-shot for about $165, gun owners in NSW for example must obtain a prohibited weapons permit if they fit a tubular magazine extension capable of extending the capacity to more than five rounds.

The Alannah and Madeline Foundation, established by Walter Mikac who lost his daughters and wife at Port Arthur, has also called on government to maintain laws enacted after the massacre, and for further restrictions on guns like the Adler A110.

"We oppose any form of gun modification allowing a firearm to be used to the same or higher capacity as a (seven-shot) Adler A110," the foundation's chief executive, Lesley Podesta, said.

More than 7000 of the five-shot version of the Turkish-made gun were imported into Australia in the past six months, as a review considers whether the type of firearm in fact undermines the National Firearms Agreement, struck in the wake of Port Arthur.