The ACT government has blamed increased patient demand, staff shortages and the lengthy process to replace old machines for the delays, which are exposed in new data from the Australian Institute of Health and Welfare.

The opposition has described the wait times as "unacceptable", fearing blowouts could put patients at increased risk.

Data from the 2017-18 financial year, published this week, showed that half of cancer patients in the ACT were receiving treatment within 21 days, while it took six weeks for 90 per cent of patients to start their first course of potentially live-saving radiotherapy.

A total of 1377 courses of treatment were delivered in that 12-month period.

In comparison, the average patient wait time Australia-wide was 10 days, for 50 per cent of patients, and 26 days, for 90 per cent of patients.

The wait time for Canberra's cancer patients has risen sharply from 2016-17 levels, when the ACT was on par with Tasmania, and better than Western Australia.

In the period, 90 per cent of patients had started treatment within 28 days - a fortnight earlier than the average patient wait time in 2017-18.

The wait time is calculated as the number of days from when the patient's doctor determines that radiology treatment is needed, to when the patient starts receiving treatment.

Weekends and public holidays are factored into the wait time, as are days where treatment cannot be delivered because staff are absent or equipment is broken.

Canberra Health Services' executive director of cancer and ambulatory support, Cathie O'Neill, said the wait times were the result of an increased number of patient referrals, and increasingly complex treatment techniques.

Ms O'Neill said two ageing radiotherapy machines - known as "linear accelerators" - were being replaced, a process which took eight months per machine

Workforce shortages were also blamed for the delays, with Ms O'Neill noting that the hospital has to recruit radiation therapists from interstate because there was no training offered locally. Nine new therapists have been hired in the past year.

Ms O'Neill said with the new machines due online in mid-2020, wait times were anticipated to return to "within national standards".

The opening of Canberra's first private radiation oncology service earlier this year was also expected to take pressure off the public hospital.

Ms O'Neill noted that all emergency radiation treatment was delivered within one day, ranking the ACT equal-first in that metric alongside Western Australia.

Opposition health spokeswoman Vicki Dunne said it was "unacceptable" that cancer patients in Canberra had to wait longer to access potentially life-saving treatment.

"We are a wealthy jurisdiction with the most expensive public hospitals in the country - why are our cancer patients worse off? Where is all the money going?" Ms Dunne said.

"Fighting cancer is an incredibly stressful ordeal for patients and their families. They shouldn't have to suffer the added stress of whether they will get treatment in time.

"My real fear is that delays to cancer treatment is putting patients at increased risk."