Kiwis can get their booster shots from Wednesday – with experts saying the extra Covid-19 dose will be critical protection against the inevitable arrival of Omicron once the borders open.

Booster vaccine doses are now officially available at walk-in vaccine clinics to an estimated 1.2 million people who received their second Pfizer Covid-19 vaccine four months ago.

Appointment bookings can be made by phone, but online bookings are not open for boosters until January 17, a Ministry of Health spokesman says.

On December 23, Covid-19 Response Minister Chris Hipkins announced the eligibility for the booster vaccine had been revised from six to four months after a person receiving their second dose.

Many Kiwis have already been taking the opportunity to get a booster, with clinics offering them according to the new four-month gap criteria, due to confusion over the implementation date.

Close to 4500 booster shots were given out on Monday, a jump of more than 1500 on the day before, according to the Ministry of Health.

While 92 per cent of New Zealand's eligible population have had two doses of the Pfizer vaccine, research to date shows protection wanes significantly within months.

Evidence shows a booster, or third dose, is effective in increasing protection significantly, the Ministry of Health says.

The Cabinet is due to meet on Friday to review the latest information on the Omicron variant.

So far there have been at least 90 cases of Omicron at the border and three in the community, the most recent a household contact of the Air New Zealand crew member who tested positive in late December.

The person was in isolation during their symptomatic period but had since been moved to a managed isolation and quarantine facility, the Ministry of Health said on Tuesday.

Otago University epidemiologist Michael Baker says Omicron poses a huge risk to New Zealand.

'We're highly vulnerable. It would behave like it's behaving in New South Wales and Victoria, it would rapidly sweep through the population,” he says.

'The risk is that it would create such an intense pandemic wave that it would be very hard to manage very ill people in hospital.”

While evidence appears to show the variant causes less severe illness, there are still many unknowns, including the risk of 'long Covid”, Baker says.

He says it will be 'useful” for the Government to set a target for booster vaccines, and suggested it would need to be at least 50 per cent of the eligible population before opening the border.

Fewer than 10 per cent of eligible Kiwis – 333,820 – have had a booster as of Tuesday, according to Ministry of Health data.

The ministry encouraged everyone over 18 who had completed their primary vaccination course and whose second dose was received four months ago or longer to get a booster dose, a spokesman said.

Baker says the Government's postponement of a decision to open the border with Australia until the end of February has given the country two months to prepare for community transmission of Omicron.

He says as soon as New Zealanders can travel to Australia, they will bring the highly infectious strain back with them, 'meaning the Omicron variant will arrive almost immediately”.

In New South Wales, cases of Covid-19 have exploded, with 23,131 new cases reported in one day, as of 8pm on Tuesday.

Cases jumped by over 85,000 in one week, from 41,738 to 126,801. More than 1300 people with Covid-19 were in hospital, with 105 in intensive care and 27 requiring ventilation, according to NSW Health.

Baker urges Kiwis to get their booster as soon as possible, saying it will provide critical protection against the much more infectious Omicron strain.

'We don't know how long we'll keep Omicron out for. At any moment there could be a border failure, and it could start circulating very widely.”

The Covid-19 vaccine will be available for children aged between 5 and 11 on January 17.

-Stuff/Cate Broughton.