A dump of Cabinet papers and official advice has revealed how difficult it was to design the gun buy-back scheme with such limited time and information.
The advice includes KPMG's report on the pricing structure, which the Police Minister and Commissioner had previously declined to release key details from.
National has been calling for the release of the report since details of the buy-back were announced in June, so gun owners could decide whether the scheme was fair.
From the outset of the report KPMG warned of the limitations in developing such a complex scheme.
"The tight timeframes and data gaps imposed a number of limitations on our ability to draw clear conclusions on how various pricing options would perform against the principles of the buy-back scheme," the report said.
The main principals KPMG were working off were the need to remove as many prohibited firearms as possible, and to fairly, but not excessively, compensate gun owners.
In June, Finance Minister Grant Robertson and Police Minister Stuart Nash announced the scheme had been boosted to $208 million and the buyback price for prohibited firearms, paid only to those with a valid firearms licence, would reflect the brand, make and model of the prohibited firearm; its base price; and its condition.
Police recommended compensation based on the condition of the firearm, with three 'price points'. KPMG proposed these price points be set at 95 percent, 70 percent and 25 percent.
Ministers agreed to the base price list developed by KPMG, and made no changes to the base, because it reflected the fair market value prior to the law change.
But the KPMG report outlines some concerns raised by firearms experts they consulted with about this three price points model.
Potential challenges identified included the scales being too granular and likely to cause high levels of under and over valuation, which could lead to greater levels of referrals to the Exceptions Panel.
KPMG also noted feedback from firearms experts that while the majority of people in the industry understand the government's need to act, "there is a lot of pain and anger in the firearms community".
"They believe the pricing needs to reflect that this is a 'distress' sale", the report said.
National police spokesperson Brett Hudson said the report backed up his concern that experts were not given much time, and their calls were not heeded.
"While they weren't asked specifically to recommend an option, clearly the commentary they gave [in the report] pointed to the option that would yield the best compliance - a standard price option," Mr Hudson said.
"New Zealanders have been told that this buyback is all about improving public safety. If that is the case then compliance should've been at the very top of the list of considerations for the government when choosing an option.
"So quite frankly at the end of that it does look like the government has chosen a less optimal outcome, presumably to save a bit of money."
However, Mr Nash said the government had struck the best balance it could in developing the gun buyback scheme.
He said the government weighed up all the information and agreed with police on the three-tier model because it was the fairest.
"You go to a five-tier model and you introduce a level of complexity which can be quite difficult.
"You go to a one-tier model where you just pay the same price for everyone and then you get some people who feel incredibly ripped off and some people who will have made a hell of a lot of money out of their firearms.
"Now, that was just never going to work," he said.
Mr Nash said the feedback he was getting was the majority of gun owners agreed the pricing was fair and they were handing in their firearms because it was the right thing to do.
"There's a general acceptance now that there is not really a place in New Zealand society for guns which are designed primarily to kill people," Mr Nash said.
Concerns were also raised in the report about the complexity of evaluating the condition of firearms when dealing with such a high volume of them.
"While applying condition to the pricing of the buy-back may theoretically offer the benefit of increasing the accuracy of any valuation, there is a risk that there is inadequate expertise to complete this work at volume and that there will be inconsistencies of assessment between different evaluators", the report said.
KPMG's report also came with a warning that without a gun register, it couldn't be confident about the potential costs of any buy-back scheme.
"The majority of data sources have lacked rigour and have been riddled with assumptions.
"These estimates have been compounded with further uncertainty because it is not clear what level of owner compliance to expect," the report said.