The Auditor-General's office is investigating the use of travel perks by former National Party MP Pansy Wong and her husband Sammy Wong.
Mrs Wong, the former MP for Botany, resigned from Parliament in December last year after misusing the international travel subsidy available to Members of Parliament.
The Auditor-General is investigating the Wongs after the Labour Party criticised an independent report which found the couple did not systematically abuse their travel perks by doing private business on taxpayer funds.
The independent investigation by senior public servant Hugh McPhail into 13 trips to China found no systematic abuse by Wongs, but did find one example where some private business was carried out which is against the rules.
The Auditor-General last year decided not to investigate further but has now changed its mind after the Labour Party provided new information.
Labour says Mr Wong is potentially involved in a new contract for 38 Auckland trains. Senior MP Trevor Mallard says the KiwiRail board is close to making a decision and Labour will ask the board to freeze the process.
KiwiRail says it has had no involvement with Mr Wong during the electric train tendering process and, to the best of its knowledge, neither have any of the vendors.
KiwiRail says it is not close to making a decision on a contract and that this is likely to happen at the end of the year.