"The rate of increase has gone from a four-week doubling time, to three weeks, and now possibly we're getting closer to two weeks – so those numbers do seem to be going up a little bit quicker."
Yet, he said, they still weren't rising at the kind of speed that would indicate a new subvariant was the biggest driving factor, as this was in previous waves.
Rather, he suspected the uptick could be a result of behavioural change, noting it began not long after the Government rolled back key controls like mask-wearing in many settings.
"The main variants we're concerned about are still growing, but [Omicron subvariant] BA.5 still makes up about 80 per cent of reported cases – so it doesn't look to be driven by a change in variants."
Given the relatively slower trajectory cases seemed to be on, he anticipated this wave's peak could be a few weeks away at least.
Covid-19 modeller Dr Dion O'Neale added that it would take time to get a clearer picture of drivers.
"[Variants] might still only be making up a small fraction of the total cases we're seeing but, depending on how much growth advantage they have over existing variants, they have the potential to become the dominant factor driving cases up," he said.
"However, it takes time to see if that's really what we're dealing with, and it's possible that the behaviour-driven growth might infect people first - meaning that there are slightly fewer susceptible people for new variants to infect by the time they would otherwise be becoming dominant."
With cases rising – and the fact those officially reported were just a portion of the true level of infection in the community – Welch said Kiwis should remain all the more vigilant.
"If you're organising any event, think carefully about how you're treating Covid-19 – and make sure the spaces are well ventilated."
Welch was also concerned many Kiwis hadn't received their booster, including hundreds of thousands of people older than 50 eligible for a fourth dose.
"If the Government's attitude is that vaccines are the thing that are going to help us, then we actually need to take advantage of them and go and get our shots."
Meanwhile, the ministry has also a reported a further 18 deaths in the past week.
Two were from Northland, three were from Auckland region, three were from Waikato, one was from Bay of Plenty, one was from Hawke's Bay, one was from Wellington region, one was from Nelson Marlborough, four were from Canterbury, two were from Southern.
As of midnight Sunday, there were 323 people in hospital with the virus, including eight in intensive care.
