Cancer diagnoses in Victoria declined in the past year, but that doesn't mean the illness is on the outer.

Nearly 2500 fewer diagnoses were made in 2020 to the previous year, but Cancer Council Victoria believes the decline is likely due to the pandemic rather than fewer people getting sick.

The Cancer in Victoria 2020 report, released by Cancer Council Victoria on Thursday, found the most common cancers that went undiagnosed were cancers of the oral cavity, prostate cancer, melanoma and bowel cancer.

Breast cancer diagnoses in females also fell in April to June but recovered slightly by the end of the year.

Professor Sue Evans, Director of the Victorian Cancer Registry, says the reduction in diagnoses is likely due to Victorians not showing up for a screening or assessment during the pandemic.

"Given the steep decline in cancer diagnoses between April and June 2020, we had anticipated seeing a higher than anticipated number of diagnoses in the latter part of 2020, but this did not occur," Professor Evans said.

Melbourne went through two COVID-19 lockdowns in 2020, with the longest lasting 111 days from July to October.

And just because they weren't diagnosed doesn't mean the cancer isn't there.

In 2020, more men were diagnosed with cancer than women. Men were diagnosed at a rate of 123 males for every 100 females.

There was a 13 per cent decline in prostate cancer diagnoses, making it one of the most undiagnosed cancers in Victoria in 2020.

Associate Professor Zee Wan Wong, head of oncology at Peninsula Health is concerned there are more than 2400 people in Victoria still undiagnosed with cancer.

"These undiagnosed cancer patients may present very late or with more complex clinical circumstances that could potentially make their care more complicated or have poorer outcomes."

Cancer Council Victoria chief executive Todd Harper said it was imperative that all Victorians prioritised their health.

"It's understandable that with state and international borders now opening up, Victorians will be eager to get on with life as soon as possible," Mr Harper said.

"If you have been invited to take part in a cancer screening program, please don't delay, please make that your first priority. With 31 deaths from cancer every day in Victoria, we must act now to avoid a major health crisis.

"Cancer screening saves lives. It is one of the most effective ways to detect the early signs of cancer."