More than one million Australians have put off seeing a doctor or specialist because they could not afford the costs in the last year.

Figures released from the Australian Institute of Health and Welfare (AIHW) today showed that 663,000 Australians were unable to cope with the cost of visiting a non-bulk billing GP in the 2016/17 financial year.

A further 538,000 were unable to afford an appointment with a specialist, while another 274,000 didn't get a pathology test or diagnostic imaging for the same reason.

Meanwhile, $3 billion was forked out among Australian patients to cover 'gap' payments for medical services.

Half of all patients were also forced to cover some of the cost of a medical appointment because it wasn't all subsidised by Medicare.

The findings come as part of an AIHW review into out-of-pocket costs for Australian patients using non-hospital Medicare services, including GPs, obstetric and medical imaging.

The report found that the median out-of-pocket costs for patients was $142, but varied greatly across the country.

For some patients in northern Sydney, those out-of-pocket costs hit $876 - six times the median.

"A range of factors can influence out-of-pocket costs," AIHW spokesman Michael Frost said.

"These can include the types of services the patient requires, the availability of bulk billing and the cost of services in their area."

Most of the $3 billion patients spent on out-of-pocket costs during the year was paid to specialists (34 percent), followed by GPs (25 percent), and diagnostic imaging services (12 percent).

But it was obstetric services that had the biggest variations in out-of-pocket expenses. Of the 3361,000 people who used out-of-hospital obstetric services, 44 percent paid part of the bill.

However the median out-of-pocket costs for obstetric patients in northern Sydney ($227) were more than 10 times that in Gippsland in Victoria ($21), while some patients in the Northern Territory forked out $887.

Nearly three-quarters of patients who saw a specialist also didn't have their bills fully covered by Medicare, spending about $64 per appointment.

For those who paid to see their GP, the out-of-pocket costs ranged from a low of $12 in western Sydney to $51 in Canberra.Median costs for diagnostic images also ranged from $25 in western NSW to $90 in Canberra.