Former chief corporate regulator Greg Medcraft will again take on the big four banks as he looks to boost competition in the mortgage sector, as he joins the board of Australian Finance Group, an $800 million listed mortgage broker.

Mr Medcraft, who only decided to return to Australia from his Paris-based role at the Organisation for Economic Co-Operation and Development in July, has hooked up with old friend Tony Gill, chairman of AFG, who he knew from Macquarie when he was working as head of mortgage securitisation at French multinational investment bank Societe Generale.