One senior Nationals MP, George Christensen, said the act of rebellion should be seen as a warning shot "fired across the government's bow" by government senators frustrated by the Coalition's own "inaction" on the issue.

The events unfolded after Liberal Democrat senator David Leyonhjelm put forward a disallowance motion which, if approved, would allow for the importation of the updated Adler model firearm, which can fire seven successive shots, up from five.

The motion was defeated because the major parties and the Greens combined to vote it down 54-7, meaning the ban will stay in place. But Nationals backbenchers Bridget McKenzie and John Williams crossed the floor to approve the motion.

Three other Nationals, including ministers Nigel Scullion, Fiona Nash and Matt Canavan joined backbencher Barry O'Sullivan in abstaining from the vote. In contrast, Liberal cabinet ministers George Brandis, Scott Ryan and Mitch Fifield were present in the chamber to vote against the motion on behalf of the government.

Sources said the decision by the Senators to cross the floor was announced to the Nationals party room meeting earlier on Monday morning. The move came after the Nationals suffered the biggest swing in Australian political history to lose the heartland state seat of Orange in New South Wales to the Shooters, Fishers and Farmers party.

Senator Canavan declined to comment and Senators Nash and Scullion did not respond to requests for comment.

Senator McKenzie, a licensed firearm owner and chair of the Parliamentary Friends of Shooting group, told Parliament those who claim allowing in the Adler shotgun would weaken Australia's tough gun laws were running a scare campaign.

"I am not arguing for a weakening of gun laws and I never have," Senator McKenzie said.

"Nearly one million Australians own at least one gun. We champion our Olympic shooters and we manage our feral pests," she said.

The Victorian senator hit back at critics of relaxing the ban and said they were wrongly conflating illegal firearms and terrorism with legitimate gun use in Australia.

"This particular disallowance motion does nothing to change the national firearms agreement or the strong gun laws which have held up in a such strong stead from a safety perspective, getting that balance right for the last 20 years," she said.

"I support the motion," Senator McKenzie told her colleagues.

Labor senator and the leader of the opposition in the Senate, Penny Wong, said the government was "now in tatters", with MPs and ministers openly rebuking the prime minister's position.

"Yet again we have disarray," said Senator Wong.

"The Turnbull government split on the floor of the Senate – the Nationals voting with Senator Leyonhjelm and where were the National cabinet ministers? Were they here?"

Nationals leader and Deputy Prime Minister Barnaby Joyce was relaxed about the split on Tuesday, saying "the National Party is its own party" which believes in the freedom of the individual.

"We are in Coalition. Similar to what Labor and the Greens are when they work together, but they are their own parties," he told ABC radio.

"[The senators] put very cogent arguments forward about what they believed the deal was in regards to the Adler, that they were respecting that deal they believe they had with Leyonhjelm and they were going to honour it."

Mr Joyce said he "absolutely" supports their right to vote independently but declined to say what his position was on the firearm ban.

'Hand forced'

The Nationals' Chief Whip in the lower house, George Christensen, told Fairfax Media his upper house colleagues had been forced into their act of defiance because of the government's inaction on the issue.

"There will be no whipping for crossing the floor in this instance. In fact, I fully endorse the stance of the Senate backbench Nationals," he said.

"The ongoing import ban on the Adler A110 is something our rank and file supporters are opposed to and our Nationals MPs and senators have been at pains to get the government to address."

"I guess the Senate backbench Nationals thought the only way to deal with the inaction was to fire a shot of their own across the government's bow, and cross the floor."

West Australian Liberal MP Ian Goodenough, who also supports overturning the ban, agreed that frustration was a factor in the Nationals insurrection, which he said he backed.

"I have had conversations with Senators McKenzie and Williams – they have a good understanding of firearms and are frustrated that their colleagues do not comprehend the absurdity of refusing to classify the Adler."

"Their decision to cross the floor is justified as they know there are many other legally available firearms of similar capacity on the market."

Mr Goodenough said state police ministers should have reclassified the Adler shotgun into Category B, which makes it somewhat difficult to obtain. He said similar lever-action shotguns and rifles had already been classified in this way.

Currently the Adler shotgun is unclassified. Nationals MPs say it would be mainly used by farmers managing feral pests on their properties. But under former Prime Minister Tony Abbott, the government agreed to put a time-limit on the ban in agreement with Senator Leyonhjelm for his vote on separate legislation.

The ban was due to expire in July but the Turnbull government continued it prompting Senator Leyonhjelm to accuse the government of reneging on their deal. Mr Turnbull initially failed to rule out fresh negotiations with Senator Leyonhjelm to secure his vote for union-busting bills, which prompted a rebuke from Mr Abbott.

Mr Abbott then tried to walk away from the original deal struck by his cabinet ministers by saying had not personally been part of the deal as prime minister.

Independent Tasmanian senator Jacqui Lambie said allowing the Adler shotgun to be imported into Australia would be a "terrorists' wet dream".

A report prepared last month by the Australian Criminal Intelligence Commission says Australia's 816,00 firearms licence holders own 2.89 million guns between them.

with Fergus Hunter

Follow Latika Bourke on Facebook