Up to 400 people could die from cancer if hospitals don't catch up quickly enough after the lockdown, the Cancer Society says.
It was basing its analysis on new British research into the impact a three-month delay in cancer diagnosis and treatment there because of the pandemic.
The study, by The Institute of Cancer Research and the Royal Marsden Hospital, showed fewer cancers were detected in the lockdown because people were not seeing their GPs or there were not as many diagnostic scans like colonoscopies or CT scans.
Cancer Society medical director Chris Jackson said New Zealand could avoid additional deaths if cancer services were resumed quickly.
DHBs needed to prioritise cancer treatment over some other conditions to try to avoid that collateral damage.
"The thing with cancer is it gets bigger and it spreads faster in a short space of time and we are very clear about what the impacts of delays are in terms of lives lost," he said.
Under the lockdown, urgent treatment had continued along with some diagnoses, but there had been fewer operations, scans and diagnostic tests.
Those now needed to resume, and there needed to be extra work to catch up, Dr Jackson said.
That should include more surgery, weekend clinics and using private hospitals and facilities to help clear the backlog, he said.
People should not delay seeing their GPs, he said.
The Cancer Control Agency was set up last year to oversee cancer care and reduce inequities.
Its chief executive Diana Sarfati said it was working with senior doctors on how to get more services up and running.
"The Cancer Agency has been providing very clear guidance to DHBs about the expectation around both cancer diagnosis and cancer treatment at this time which would ensure that people who need care are getting it," she said.
Screening programmes for breast, cervical and bowel cancers would start once colonscopies are up and running.
Director-General of Health Dr Ashley Bloomfield said getting normal hospital services back up and running could not be done at the expense of Covid-19 preparedness.
Hospitals still needed to be ready to respond quickly if the disease flared up again.