Roger McClay is being investigated by police for allegedly using his former-MP perk of taxpayer-funded flights on business trips and then claiming driving mileage from the charity he headed.
No charges have been laid and he has denied any wrongdoing.
Detectives executed search warrants on the Auckland home of the ex-National Party MP in June and seized financial records from Keep New Zealand Beautiful, of which he was the chairman.
Former staff and fellow board members have also been interviewed by police.
Police have also obtained the records of Heart Children New Zealand and detectives have spoken to senior staff members.
Lara Syddall, chief executive of Heart Children, confirmed documents had been handed to police.
She said Mr McClay worked as a contractor in a government liaison role but declined to comment further.
Mr McClay performed a similar government liaison and education role at World Vision.
Spokesman Phil Johnstone would not comment on whether the police had obtained World Vision records or spoken to staff.
Two other charitable trusts Mr McClay is involved with - Variety, the Children's Charity and For the Sake of Children - said they were not part of the police inquiry.
Mr McClay (64) is under investigation for allegedly invoicing Keep New Zealand Beautiful for the cost of driving mileage, but taking a taxpayer-funded flight instead.
As a former MP, Mr McClay is entitled to the 90% subsidy for 12 domestic return flights each year.
No charges have been laid and he has denied any wrongdoing.
Mr McClay said he was disappointed at the length of the police inquiry but declined to comment further.
Mr McClay resigned as Keep NZ Beautiful chairman in July to take up a position at the NZ Spinal Trust.
During his time in Parliament, until 1996, he held positions as minister of Youth Affairs and associate minister of Education and Social Welfare.
He was then appointed the Commissioner for Children and in 2005 was made a companion of the Queen's Service Order in recognition of his public service and contribution to the welfare of children.
A source said the Parliamentary Service, which administers MPs' travel subsidies, spoke to staff at Keep NZ Beautiful last December before a complaint was laid with police.
The police inquiry was headed by Detective Senior Sergeant Greg Cramer, who has since moved to the North Shore as a detective inspector.
Spokeswoman Noreen Hegarty said police did not "disclose the identities of people under investigation".
A spokesman for the Parliamentary Service refused to answer questions about Mr McClay's travel claims.
