The lockdown protected cancer patients - but test delays are now threatening hundreds of them.

Doctors are racing to clear a backlog of tests after cancer cases dropped 47 per cent, with 1031 fewer registrations in April from a year ago.

Cancer Society medical director Dr Chris Jackson told Mike Hosking the focus has gone from shielding cancer patients from Covid-19 to treating them.

He says delaying cancer treatment over the next three months could result in 400 deaths.

"The thing is how do you recover now and look after people who's diagnoses were missed or delayed, and we have to do that relatively quickly."

Jackson says some cancers can be slowed down, but a six-week delay for other types can have a big impact on mortality.

"If you delay your diagnosis by up to three months for cancer, you do get an increase in deaths. It's different for different cancer types, if you model that out on average, that's about what you'd get, about 400 in the New Zealand population."