The report Cabinet relied on to form its decision around the gun buyback programme was not "robust enough" to provide an accurate picture of how much the scheme would cost.
The Government today released a tranche of information it issued to inform its decision on the programme.
So far, more than $12 million of compensation has been handed out to 4200 Kiwi gun owners who have surrendered their firearms at collection events across the country.
After the March 15 terror attack, the Government set aside $200 million for the gun buyback scheme.
But a report by KPMG, commissioned by the Government, revealed was a series of unknown factors when it came to quantifying the buyback programme.
The Government settled on a scheme whereby gun owners would be paid based on the condition of the gun they were surrendering.
The owner of a gun in poor condition would get 25 per cent of its base price, 70 per cent for a firearm is average condition and a 95 per cent for fun that was in near-new condition.
But because officials have no idea how many guns are in New Zealand, estimating an accurate final cost was problematic, according to KPMG.
"It is not clear whether the majority of prohibited firearms are at the midpoint of a price range, nearer the top-end or nearer the bottom-end."
And based on conversations with NZ owners and dealers, and looking at US data, KPMG expected most military-style semiautomatic weapons to be high-value rifles which will be in excellent, very good or good condition.
"We understand from firearms experts that there may be some very rare and highly collectable firearms that are now prohibited that may be have values well over $10,000, even as high as $100,000."
If the average amount of money that is paid to a gun owner surrendering their weapon was $4330 – the potential cost of the buyback could as high as $752 million, KPMG's report says.
KPMG said the financial observations made in its report might be useful for "option comparison purposes".
"However, KPMG does not believe they are robust enough to fully provide confidence in the financial implications of any of the buyback pricing options presented."
Police Minister Stuart Nash said the Government has consistently said it will top up the provision for the buyback if required.
Finance Minister Grant Robertson said: "There is high uncertainty around any costings, owing to the lack of information on the number of prohibited items, their type and condition."
But he said better information will be forthcoming once the buyback is under way and volumes and conditions of firearms are clearer.
But ACT Party leader David Seymour said the report showed the Government made critical decisions about its gun buyback based on numbers that significantly underestimated the cost to taxpayers.
"The Government should have done its homework before deciding to ban hundreds of thousands of guns, but Jacinda Ardern was too eager to announce to the world's media that she had done so."
The total number of all weapons, both lawful and prohibited, is estimated to be approximately 1.2-1.5 million.
Police records show approximately 14,300 military style semi-automatics are registered against licence-holders.