The government has proposed saving $270 million by changing the Medicare safety net, which covers out-of-hospital services such as in vitro fertilisation, specialists, and some pathology and diagnostic imaging services.

The increased thresholds, due to come into effect in January 2016, mean patients would need to spend more on out-of-pocket medical expenses to become eligible for additional Medicare benefits.

David Molloy, chairman of the Fertility Society of Australia, said the proposed changes would affect chronically ill patients who require regular treatment and high-cost medical interventions. Affected patients would include cancer patients, fertility patients and those who need regular access to psychiatric services.

The cost of IVF treatments could rise by up to $1800 a year, he said.

Dr Molloy said legislation for the Medicare safety net changes was expected during the current sitting period, but was postponed after the death of longstanding Liberal MP Don Randall in July.

The Canning by-election is seen as a critical test of Prime Minister Tony Abbott's leadership.

"Coalition MPs have said privately that the changes have been held back because they don't want to fight on the issue during the Canning by-election," Dr Molloy said.

"This is one of the most significant attacks on the fundamental structure of Medicare since its inception. Before the election we took the government at their word that there would be no cuts to health."

Australian Medical Association President Brian Owler said: "I was told the legislation was due to be introduced a few weeks ago, but nothing happened."

The Canning by-election could well explain the delay, he said.

Dr Owler said the AMA does not support the proposed changes: "This is just another way of the government shifting costs onto patients."

The changes were originally contained in the 2014 budget and, unlike the dumped $7 Medicare co-payment, carried over into this year's budget.

Government sources denied the legislation was being held back because of Canning, noting that it was originally introduced by former health minister Peter Dutton and would be difficult to pass through the Senate.

Health Minister Sussan Ley has been consulting widely with the medical profession on Medicare, including changes to the safety net, they said.

Ms Ley said the safety net changes were aimed at making the current system "easier to access and understand" for patients.

"The current system is overly complex and this measure aims to ensure more people can access the protections of the safety net sooner if they get sick," she said.

"As I announced earlier this year, the government is currently working closely with patients and health professionals to build a healthier Medicare and this outstanding 2014-15 budget measure is currently being considered in this context, with consultations ongoing."

Labor health spokeswoman Catherine King said: "These cuts will hurt some of our most vulnerable patients.

"The people of Canning deserve to know what cuts Tony Abbott has in store before they go to the polls this Saturday."