More than 100 witnesses will be called to give evidence against former federal independent MP Craig Thomson as he fights fraud charges.

Thomson is fighting 173 charges over the alleged misuse of his union credit cards while he was the national secretary of the Health Services Union.

It is alleged he spent $28,000 of union money for personal use, including paying for escort services, adult movies, hotel stays and flights.

The Melbourne Magistrates Court heard 113 witnesses will be called to give evidence over whether it was Thomson using the cards, and if he was authorised to do so.

The hearing will run for more than a month.

Thomson's lawyers are challenging whether he had authorisation to use the cards.

If they are successful, the case against him will be thrown out.