"It's a shoddy process, in a shoddy way by a shoddy Government," National's shadow leader of the House Chris Bishop said.
In August last year, Parliament passed a bill "in a hurry", according to Covid-19 Response Minister Chris Hipkins.
The legislation allowed the Government to charge people for staying in managed isolation.
After MIQ charges were introduced in August 2020, there was a carve-out for New Zealanders returning home, as well as Australians who lived in New Zealand.
However, visiting Australians were being charged for MIQ. The Government recently learned that may have been unlawful, because under immigration law, Australians automatically got residents visas when they entered New Zealand, regardless of the visa that they travel on.
That meant that technically it could be argued that there was no lawful authority to charge those Australians. The bill was a move to retrospectively change the law, so that those charges were lawful and could not be challenged.
The quarantine-free travel of the travel bubble means it is no longer a major issue - but it could come into play again if an outbreak results in visitors having to go into MIQ again.
National's Chris Bishop also argued that it was inappropriate for the Government to use Budget urgency to make the change.
Legislation to bring in the changes to benefits and allowances that were the key announcements in the Budget passed early this morning.
Hipkins was not able to say exactly how many were incorrectly charged, but estimated it was between 600 and 650 people.
The bill fixes this error but in order to right the wrong, MPs were forced back into the House to pass the amendment to the law under urgency.
Parliament does not sit on Friday so the urgent sitting was unusual.
"This is being done now, under urgency, to address any uncertainty about how Australians have been charged," a spokesman for Hipkins said.
"This Bill is purely technical, and does not change the policy position on who is liable to pay MIQ charges and the level that they are required to pay."
But Bishop said the second time this week that the Government has used urgency to change a law retrospectively – "it is a shoddy process".