"If that continues, we would be seeing around 160 new cases per day by early November."
Dr Plank and others predict strain on New Zealand's contact tracing, quarantine and health systems from the growing caseload.
University of Otago epidemiologist Amanda Kvalsvig said "it's time to talk about a circuit-breaker".
"A move back to alert level four is the best, and probably only, chance of reversing these highly concerning trends," she said.
Dr Kvalsvig said Maori, Pacific people, children and those from vulnerable communities are more likely to catch COVID-19, with worse outcomes, and their voices needed to be heard.
"This is the most urgent and most important national conversation we can be having right now," she said.
"At this critical point in our pandemic journey, failing to act decisively will have severe consequences for population health and wellbeing."
While cases are low by global standards, the Delta outbreak identified on 17 August has now overtaken New Zealand's first outbreak to be the biggest of the pandemic.
Until the arrival of Delta in mid-August, New Zealand had curbed the virus through its elimination strategy, experiencing more days without COVID-19 restrictions than with them.
Those days appear gone.
Health officials have stopped reporting the active sub-clusters in Auckland, with 75 unlinked cases identified in the last fortnight.
"What that means is there's a lot more cases out there," University of Auckland epidemiologist Rod Jackson told Radio NZ.
"Delta just spreads so quickly, so widely ... that it just overwhelms all services.
"[Quarantine] is going to be overwhelmed. The next thing is hospitals are going to be overwhelmed. Everything is going to be overwhelmed. We've just got to slow it down."
Auckland moved to an alert level four lockdown on 18 August, reducing to level three on 22 September - a shift which has coincided with rising cases.
COVID-19 Minister Chris Hipkins ruled out a return to level four, saying Aucklanders were less likely to comply with stricter rules after 58 days in lockdown.
Dion O'Neale, principal investigator at cross-university research group Te Punaha Matatini, said level four would buy the government time to up vaccination rates.
"In such a situation it is very tough to vaccinate our way out of the growing case numbers," he said.
"There's a strong case to be made for a temporary return to alert level four in Auckland as a circuit-breaker to limit transmission for a few weeks while we try to get as many people vaccinated as possible."