(CNN) New Zealand's proposed buyback of semi-automatic firearms could cost as much as $300 million NZD ($200 million), Winston Peters, the country's deputy prime minister, said Tuesday.

In the wake of the Christchurch terrorist attacks on March 15 which killed 50 people , New Zealand has acted swiftly to change its gun laws.

On Tuesday, a bill which proposes banning semi-automatic firearms, magazines and parts used to assemble illegal firearms passed its first reading in Parliament, gaining the support of every lawmaker in the house, bar one.

If the bill passes, gun owners will have an amnesty until September to hand in their weapons and be compensated as part of a proposed buyback scheme.

The government had previously said the proposed buyback could cost between $100 to $200 million NZD ($68 and $136 million), but on Tuesday morning Peters revised the estimate to $300 million NZD ($200 million).

"Now, this is an extravagant statement but it may well be true -- it could cost up to $300 million to set what's wrong right," he told CNN affiliate Radio New Zealand (RNZ). He admitted the government was unable to assess the full cost of the scheme, as it didn't know how many weapons, or what kind, it would need to buy back.

In New Zealand, gun owners require a license, but do not have to register their weapons, meaning no one knows for certain how many guns are in circulation.

The New Zealand Police estimates that there are around 1.2 million firearms in the country. As of Monday, 211 firearms had been handed in to police, with some gun owners even doing so before the proposed law changes had been announced, according to a statement from New Zealand Police.

But Nicole McKee, the secretary of the Council of Licensed Firearms Owners, disputed the government's buyback sum, telling RNZ that the law change would cost taxpayers "billions."

"We are very concerned about the unfair and unjust treatment of the New Zealand firearm owners," she said on Tuesday, adding that it would make the 250,000 "law-abiding" citizens who are licensed firearm owners into criminals.

petition opposing the proposed changes has now gained almost 15,000 signatures.

Australia also introduced a gun buyback after a shooter killed 35 people in Tasmania. The country's then-government recovered about 640,000 prohibited weapons, according to the Australian National Audit Office.

Swift changes

The public will have two days to make a submission on the bill -- a much shorter period than usual -- before it goes back before Parliament next Tuesday for its second reading.

Peters has said that he expects the gun changes to come into effect by April 12, four weeks after the Christchurch terrorist attack.

The sole opposing voice in parliament on Tuesday was David Seymour, the leader of ACT, a right-wing minority party.

"I urge caution," he told Parliament. "I urge public consultation. I urge that we do our job as a parliament and truly honor the victims of this tragedy by defending our democracy."

Kelvin Davis, a member of parliament who is part of Prime Minister Jacinda Ardern's governing Labour Party, said he had handed in his own semi-automatic rifle to the police the Tuesday after the attack.

"The point is that the safety of New Zealanders is more important than my need or my right to own a semi-automatic .22," he told lawmakers.

"We can't say with certainty what the cost of the buyback will be ... But then again, what's the cost of an event like we saw on 15 March?"