A Covid-19 modeller says alert level restrictions may need to be tightened again if cases jump quickly.
University of Canterbury Professor Michael Plank told Morning Report it's clear cases in Auckland are trending upwards.
"It looks like the R number has been greater than one since the shift to level 3, in fact, and we think it's about 1.2, 1.3 at the moment."
He said small shifts gradual trend upwards is okay if the strategy is to vaccinate to control the outbreak, but greater restrictions would be needed if cases started to rise rapidly.
"If cases were doubling every eight days, which is what we would expect if the R number was about 1.5.
"That would very quickly lead to an enormous number of cases, and the number of people that would require hospital treatment would put an immense strain on our healthcare system.
"So if we see the R value creeping up into that 1.4, 1.5 territory, I think that would be a sign that we need to act."
Professor Plank said it was difficult to point at one single reason why cases were going up, but the move to alert level 3 in Auckland had helped the spread.
"It's obviously a combination of the alert level restrictions, and the particular populations and communities that the virus is spreading in.
"So, it's difficult to pin it down to one single cause, but certainly, the shift to alert level three, I think, has increased the R number somewhat.
"Obviously we've seen an increase in the number of mystery cases and so on, which I think is a reflection of that as well."
