The cost of New Zealand's buyback scheme for newly banned semi-automatic weapons could balloon to $500 million, according to the Council of Licenced Firearms Owners (COLFO).

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That figure is more than double what the Government has estimated.

Prime Minister Jacinda Ardern announced the ban yesterday, in direct response to the Christcurch mosques terror attack.

Nicole McKee of COLFO said their estimate for all semi-automatics "across the board in New Zealand...would be somewhere around $500 million".

The Government's official estimate is up to $200 million.

Military style semi-automatics can cost from $200 to more than $10,000 and there are at least 15,000 registered in New Zealand, according to police figures.

However, other firearms can be modified to reach the same firepower.

The Prime Minister yesterday said "we are very much in the dark as to how many of these are in circulation".

COLFO, which represents about 30,000 people, wants compensation to be widened.

"There's also things like ammunition, reloading equipment, cases, a whole lot of things associated with people owning those types of firearms for their legitimate reasons," Ms McKee said.

After last week's terrorist attack in Christchurch that killed 50 people, gun owners began handing their firearms back to police.

The advice now is to register online with police.

Next week, ministers will consider further changes, including a potential gun register.

"Registration has failed in Canada and it cost the taxpayer $2 billion," Ms McKee argued. "Registration in Australia since Port Arthur has not been that effective either, so do we want to bring in a regime to New Zealand that has proven overseas to not be effective?"