Screening for some cancers can resume under a level 3 lockdown, but others will remain on pause, the Cancer Control Agency said.
Since lockdown began four weeks ago screening programmes have been halted, meaning those who may have cancer cannot have it detected.
Listen to the full interview
Agency chief executive Professor Diana Sarfati told Nine to Noon that under alert level 3, which begins next week, breast and cervical screening can restart, but procedures to diagnose bowel cancer would remain on hold.
Many cancer treatment regimes have been changed in order to minimise patient time in or at hospital.
Listeners have contacted RNZ with their own experiences. One said a family member's surgery for aggressive prostate cancer was cancelled because of the lockdown and was yet to be rescheduled.
Another said their son had an aggressive cancer which did not respond to chemotherapy and radiation, leaving surgery as the only option.
"He had a specialist appointment at Waikato hospital two weeks ago and was told under normal circumstances, the suspicious growth would be removed, but with Covid-19, he would have to wait until the 'dust settles'.
"We don't know how long this will be, so he's left waiting and worrying."
Professor Sarfati said screening for some cancers was not able to resume because of the different components to the process.
"Bowel screening is a bit more complicated because we've got to make sure the colonoscopy services are up and running.
"The DHBs and Ministry of Health are working together to make sure all those parts of the screening services are working well, so that includes colonoscopy services, in order to provide safe screening."
The agency, health boards and clinicians were preparing for an expected backlog of cases, Professor Sarfati said.
"Both at the national level and DHB level, and we're all clear we're going to have to use all our available capacity and be quite innovative in our approach to making sure patients get all the care they need."
She said doctors would be prioritising their waiting lists to ensure those who needed care now were prioritised.
Professor Sarfati said the sector understood that diagnosis and treatment was bad enough in normal times, but those handling a diagnosis and treatment were now more alone and in need of extra help.
The Cancer Society is dealing with more calls for support, and was now calling for help itself. Many of its volunteer drivers, who helped patients get to treatments, were aged over 70 and had therefore been stood down.
The society is in need of younger drivers to help.
Professor Sarfati said the current situation was exacerbating the level of mental stress endured by cancer patients.
"Psycho-social support is being put in place. For example, there are social workers and psychologists available to talk to patients, the Cancer Society is doing a fantastic job and we need to just keep making sure we are providing support to patients when they need it.
"Cancer doctors and nurses are also working really hard to make sure people are getting the support they need."
She advised those undergoing cancer treatment during Covid-19 restrictions to stay home wherever possible, to not stop taking cancer medicine without talking to their doctor, and if unwell, patients should contact their GP, primary care provider or cancer doctor, as hospital remained the safest place to be for those needing extra care.
The agency published this video yesterday.