"So it is significant but it is not something that sweeps around the world pretty fast.
"In terms of severity, we would expect to see similar levels to what we saw with BA.1 and BA.2 - less severe than Delta and on a par with the original Wuhan strain.
"They are not the really big jumps that we saw between Delta and Omicron, these are the smaller tweaks. But it's definitely the sort of thing we've got to keep an eye on."
Professor Michael Baker, from the University of Otago, said early indications were that BA.4 had "reasonable fitness" and could be able to "out-compete" other sub-variants.
"The fact that it has emerged in South Africa and is spreading there suggests that it does have some advantage over BA.2, and some ability to evade existing immunity.
"Because there's such a high level of immunity in South Africa already - it obviously has some competitive advantage."
The biggest concern with new variants was that they were slightly better at evading New Zealand's existing immunity, Professor Baker said.
"This will increase the risk of what we call breakthrough infections, which are reinfections essentially.
"People who have been vaccinated or have had prior infection or both suddenly start getting the infection again. That is going to be more likely if they are new variants because they will be a bit more different from the viruses we had before."
Experts said South Africa had a very different immunity profile to New Zealand, because New Zealand had a much higher rate of vaccination. Officials may glean more useful information from watching how BA.4 played out in Australia.
Baker noted that New Zealand was previously able to "sit back" and watch how new variants developed because our closed borders meant they did not arrive here for months.
But as the country continues to open up - visitors from visa waiver countries could enter from tonight - the variants could emerge in New Zealand with only a slight delay, meaning less was known about them.
The Ministry of Health said the BA.4 variant was detected at the border through whole genome sequencing which was part of ongoing border surveillance for emerging variants.
The person carrying the sub-variant followed all testing and reporting requirements, which allowed the sub-variant to be picked up quickly, the ministry said.
The arrival of the sub-variant was not unexpected, and at this stage the public health settings would not need to be changed.
"It can take weeks or months to identify the severity of each new variant or sub-variant, so the Ministry of Health will continue to monitor the emerging evidence closely."
Meanwhile, on Sunday, 5656 new cases of Covid-19 were recorded in the community - and another six people with the virus have died.
The six deaths reported today included people who had died over the past three days.
Three of the dead were in the 70s and three in their 80s.
Of the deaths reported today, one person was from Northland, one from Taranaki, two from Waikato, and two from Canterbury.
There are 466 people in hospital, including 16 in intensive care.
