About 14,000 New Zealanders are known to have been infected with Covid-19 twice and Ministry of Health figures show 183 people have had the virus three times.

The ministry says about two-thirds of the re-infections happened between one and three months after a first infection.

The figures relate only to those actually reporting their positive tests, so the number is likely to be more.

The number will almost certainly increase as people's immunity from the booster or previous infections decline over time.

Te Whatu Ora's director of population health for Counties Manukau, Gary Jackson, says new variants are also playing a part as they are able to evade immunity acquired from earlier variants.

"But Omicron can even evade itself so after about six months you have got a reasonable chance of catching the same strain."

It's now about three months since the big Omicron peaks of March and April and that could mean a surge in the numbers getting re-infected, he says.

That means it's important to keep being careful and taking protective measures like wearing masks in high risk settings and staying home if sick, he says.

Getting the fourth booster will also help, Jackson says.

A Ministry of Health spokesperson says there's still a lot to learn about Covid-19 re-infection but there's no evidence yet to suggest people will get a more severe infection the second time.

Health experts say getting re-infected increases the chances of experiencing complications like Long Covid.

-RNZ/Rowan Quinn.