Family history search firm Ancestry says researching MP citizenship does not have to be an expensive exercise.

Prime Minister Malcolm Turnbull argues an audit of members of parliament's backgrounds would cost "tens of millions of dollars" and a "huge audit office" would be required.

Ancestry told AAP on Wednesday that while it was a subscription service, many public libraries such as Canberra's National Library of Australia provided free access to the family history search service.

"So it doesn't need to be an expensive exercise," the company said.

"Family history has always been important. Understanding where you come from can help shape your future, and the current situation some politicians find themselves in is a great example of this."

Ancestry recommends a three-step process to start a family history search: start with yourself and work backwards; talk to the oldest person in the family and ask them what they know; jump on to Ancestry online to find the records to validate your research.