A survey of 2500 patients found those who were bulk-billed were most likely to have a chronic disease and/or a lower income.

Patients who have short GP appointments are no more likely to be bulk-billed than those who have longer appointments, a study has found.

The survey of nearly 2500 Australians, published in the Medical Journal of Australia, found there was no association between bulk-billing and the duration of the GP visit.

The findings come as the federal government prepares to cut Medicare rebates for standard GP visits by $5 from July, a cost that doctors may choose to pass on to their patients. The change will not affect those with concession cards or children under 16.

The survey found that 71 per cent of respondents had been bulk-billed for their most recent GP visit, saving them $34.

"There was no association between bulk-billing and duration of GP visit, age or sex," the researchers from University of Technology Sydney concluded.

They found that patients were more likely to be bulk-billed if they had a chronic disease and/or held a concession card.

Patients were less likely to be bulk-billed if they had a higher household income or attended a practice in a regional area.

The researchers said a surprising finding was that those with private insurance were more likely to be bulk-billed.

That could be explained by healthier individuals being more willing to discriminate between practices and better able to find bulk-billing GPs, they wrote.

The researchers said the current debate about increasing fees and reducing the Medicare rebate need to be approached with caution.

The groups that would be mainly affected by additional payments would be those with chronic disease and lower incomes, they said.