The federal health minister was publicly pledging her "firm support" for existing private health insurance rebates while seeking advice on means testing the payments behind the scenes, a Senate inquiry has been told.

The government announced plans to means test the private health insurance rebate in the May budget.

Health Department officials told a Senate hearing on Wednesday the move will prompt 40,000 people to dump private health cover and an extra 8,000 over two years to join the public hospital queue.

Health Minister Nicola Roxon said on February 24 the government was "firmly committed" to retaining existing private health insurance rebates.

The Department of Health and Ageing advised Ms Roxon on options to change the rebate on January 12, 2009, after receiving a request from her office.

"If you are firmly committed to retaining existing arrangements why would you be seeking advice on how to change existing arrangements?" Liberal senator Mathias Cormann asked during the hearing.

"You are firmly committed until you change your mind but in the meantime you do all this homework in the background on getting yourself ready to change your mind - is that the modus operandi?"

Parliamentary Secretary for Health and Ageing, Senator Jan McLucas, defended the comments Ms Roxon made in February.

"Until a decision is actually made it is inappropriate for any member of the government to speculate on what may or may not occur," she told the hearing.

"Minister Roxon was absolutely accurate ... when she said that we were committed to the (rebate) as it was."

Senator McLucas said the health minister was wise to seek information on different budget options considering the difficult economic climate.

Department official Penny Shakespeare said 40,000 people were expected to opt out of private health policies when the rebate means test comes into effect.

Some 25,000 are expected to drop their hospital and/or general treatment cover, she said.

An additional 10,000 people will keep their hospital cover but drop general treatment cover and 5,000 people who have a general treatment policy only will drop that cover.

People are not expected to downgrade their cover to cheaper policies due to the changes.

"The government doesn't actually expect that people will downgrade their cover as a result of this measure," Ms Shakespeare said.

She said research showed security was the main reason more than half of policy holders bought private health insurance.