West Australian finance and transport minister Dean Nalder is being investigated over a possible conflict of interest with his shares in a car-leasing company, according to a report by the Australian Associated Press (AAP).

Nalder is the director of a private investment company that has a 27.5-percent share in Fleet Network, which leases cars to public servants through salary packaging arrangements, according to the AAP report.

He also oversees the WA Finance Department, which is responsible for policies relating to salary packaging arrangements for public servants and the state government's car fleet, according to the report.

Premier Colin Barnett has ordered a review into Nalder's declaration of interests to ensure he has followed the ministerial code of conduct. Nalder said in a statement on Saturday he had sought advice from the Department of Premier and Cabinet, the cabinet secretary and the clerk of parliament about his shareholdings.

According to the AAP Nalder has excused himself from any discussion or decision-making about the state government fleet, with the responsibility falling to Treasurer Mike Nahan's office.The transport minister says he is also seeking to divest his interest in Fleet Network.Opposition transport spokesman Ken Travers said Mr Nalder should stand aside from ministerial duties while the matter was investigated.