Screening clinic closures and patients putting off seeing their GP due to coronavirus fears have likely led to the decline, University of Newcastle Professor Stephen Ackland said.

"It is fairly widely known that in the month of April, Medicare claims were down by about 40 per cent," the Lake Macquarie Private oncologist said. "To me that suggests patients were not going to see their GPs because they were fearful of being out in the community."

He said in the past two-to-three weeks medical oncologists had experienced a downturn in the number of new cancer referrals.

There was a delay because a patient would typically go to screening or to their GP, get some tests, then see a surgeon before they would be referred to an oncologist.

"I don't have official numbers, but it's obvious to most of us," Professor Ackland said. "I would have said it's down at least 30 per cent."

Patients who were at all "symptomatic" should consult their GP.

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"I think the risks of a delayed diagnosis of a malignancy - or for that matter a delay in being reassured that they don't have malignancy - probably outweigh the risk of developing coronavirus infection at the present time," he said. "While the fear of the virus is completely understandable, GPs are doing a lot of things - like telehealth - which is a safe way to get medical advice when you're at home. There is no substitute for a clinical examination, however."

Oncologist Nick Zdenkowski, a specialist at The Breast and Endocrine Centre at Lake Macquarie Private, said he had also seen a decline in the number of new referrals.

BreastScreen NSW had briefly closed towards the end of March, and about 50 per cent of breast cancers were detected by screening. The other 50 per cent were detected by a patient noticing symptoms, like a lump, and seeing their GP.

"My clinics have slowed down, and I think it will be similar with other screen-detected cancers like colon cancer," Dr Zdenkowski said.

"For screen-detected cancers, we hope the delay of a couple of months isn't going to be such a big deal, because they tend to be slower growing, but not always.

"The idea behind screening is to identify cancers at an earlier stage so they can be treated with better cure rates and a better prognosis. If things are allowed to grow a bit more, they may present at a slightly later stage."

Cancers typically identified by GPs, such as lung cancer, were likely to be more problematic as they could progress more rapidly.

"There will be some catching up to do. I think there will be a wave of new diagnoses that will come through," he said.

Data from Sydney and Melbourne had shown a 25-to-40 per cent drop in new patient referrals to tertiary cancer centres.

"I suspect it would be similar locally," he said.

"There probably will be some delayed diagnoses... Whether we can say people are going to do worse as a result, I don't know."

People should be "cautiously optimistic".

"Get any symptoms checked out," he said. "But still be aware of coronavirus restrictions and take care."