Prime Minister John Key says there is nothing untoward in ministers accepting corporate hospitality from the Government's banker, Westpac.
Green Party co-leader Russel Norman says the hospitality raises the question of a potential conflict of interest when the Government is considering putting its banking contract out to tender.
Answers to a series of written parliamentary questions have revealed that nine ministers and the staff of 13 ministers have enjoyed Westpac's hospitality in the past year, most often in a corporate box watching rugby or a concert.
Mr Key said on Wednesday that has had no influence on the Government's decision to put out to tender the banking contract held by Westpac - for the first time in more than 20 years.
Ministers are aware of the obligations they face when accepting gifts or corporate hospitality, he said.
However, Dr Norman is sceptical, saying ministers should be more open about which corporations entertain them.
One way to avoid suspicion is for the Cabinet Manual to be more explicit about accepting gifts and for them to be made public on a regular basis, he said.
Dr Norman acknowledged that other corporates provide entertainment for ministers and their staff, and that the practice happened under previous governments.