Acting Prime Minister Grant Robertson stands by Managed Isolation and Quarantine (MIQ) remaining until February despite top health official's advice that the system was no longer justified in November last year.

Robertson told Newstalk ZB's Tim Dower that MIQ would have ended in January as initially planned had it not been for the arrival of Omicron in New Zealand.

The highly transmissable variant became a greater threat to the country, Robertson said, and the extended MIQ period meant vaccination rates and booster rates could be increased.

"Had we moved quicker we would have had a significantly harder job bringing MIQ back."

In November last year director of public health Dr Caroline McElnay wrote to director general of health Dr Ashley Bloomfield seeking his agreement to an updated Public Health Risk Assessment.

She wanted it to reflect that "the risk posed by international arrivals transmitting Covid-19 is no longer higher than the domestic transmission risk of Covid-19".

McElnay asked Bloomfield if he agreed the risk was no higher, and if so: "Managed Isolation for border returnees would no longer be justified on public health grounds as the 'default' for people travelling to New Zealand," the document said.

Bloomfield did agree, and he also agreed to brief Covid-19 Response Minister Chris Hipkins, and start creating a plan for making "self-isolation" the default requirement for returnees.

The memo noted the Government might need to speed up its plan for a phased easing of border restrictions in the first quarter of 2022.

Robertson stated that before the Government was able to make a decision on MIQ, Bloomfield had taken the advice to other health experts to be peer reviewed.

"They wanted to make sure for such a significant decision, given the important role MIQ had played in protecting New Zealanders and in saving lives, as they knew they couldn't go back from that decision," said Robertson.

The advice received from these other experts was that if the Government decided to end MIQ it would have to be done carefully and considered - about a week later Robertson said the announcement to end MIQ in January was made.

It was not until March 2, 15 weeks after the initial advice was given in November, that the Government lifted MIQ requirement for inbound travellers.

There were seven more voucher lotteries and nearly 40,000 had stays in an MIQ facility during this period.

Robertson said that people were already challenging the Government's MIQ decisions.

"There are already court cases about that and we'll wait for the outcome of those but I continue to believe MIQ did a significant job in keeping New Zealanders safe and in saving lives."