Breast cancer sufferer Claire Simpson has placed herself in a self-imposed lockdown due to ongoing COVID-19 infection rates.

However, her concern isn't only about getting sick.

"It's more that if I were to catch it, it would interrupt my treatment, so there's a bit of extra anxiety," she told AAP.

"Basically, you become a bit of a hermit, you're in an isolation of your own devising."

The 52-year-old, from Melbourne's Ferntree Gully, was diagnosed with cancer after attending her first breast screening in December 2020.

She had tried to get screened months earlier, in August, but COVID-19 restrictions made getting an appointment difficult.

"And then I was called in for further tests and I was thinking, 'this is escalating quite quickly'," she said.

She was booked for a mastectomy, where specialists discovered she had invasive cancer in her breasts.

Following the procedure, Ms Simpson received chemotherapy for several months until June 2021.

Ms Simpson is awaiting further reconstructive surgery to her breasts, which has been postponed to mid-2022, due to the pandemic's impact on Victoria's health system

"I have to have another little operation and that's been put off multiple times," she said.

Ms Simpson is receiving immunotherapy treatment at a private hospital and has recently noticed a change in staffing levels due at to COVID-related absences.

"The last few times I've been, there's not been so many familiar faces. Of course everyone ends up on furlough because of exposures," she said.

"If someone was starting treatment, I would be really feeling for them in that environment because they don't have that consistency that I had when I had my chemo."

She has also struggled with visitor restrictions at hospitals, as family and friends have often been banned.

"My mum in particular has wanted to come with me because she had breast cancer 10 years ago and was keen to support me while I was having the treatment," she said.

"The medical staff are fantastic in the hospital, but sometimes it's just nice to have someone talk to you and distract you."

Friday is World Cancer Day and Cancer Council Victoria's Amanda Piper said callers to the charity's support line are feeling more isolated than ever.

"People affected by cancer are worried about the impact COVID is going to have on their treatment," she told AAP.

"And they're also feeling isolated from their support network because they don't want to contract COVID and put their cancer treatment at risk."

Cancer diagnosis rates in some parts of Australia declined in 2020 due to the pandemic.

Ms Piper said lack of access to screening programs and lockdowns had contributed to the decline.

"There were long periods of lockdown where people may have been feeling vulnerable and not going to regular checks or not raising any symptoms that were worrying them," she said.