Police have launched an investigation into the prison guards' union with a raid on the organisation's offices, the Herald on Sunday has learned.

There has also been a separate request by officers investigating finances at the union to the Department of Corrections for records relating to one of its staff.

The prison guard they have sought information on also works for the Corrections Association of New Zealand as an official.

No representatives of the union - which collects about $600,000 a year in fees from members - would comment on the investigation yesterday.

The Department of Corrections was also silent. Organisational development general manager Vincent Arbuckle said: "The department does not comment on individual employment investigations.

"The department takes allegations of inappropriate conduct very seriously and fully co-operates with police on any criminal investigation they undertake."

The police inquiry comes after a complaint made by senior prison officer Helen Williams, 45, over spending controls at the organisation.

Williams said she had repeatedly raised concerns at a national level within the union with the backing of the Northern Regional Corrections staff she represented.

However, she said her concerns had been constantly rebutted by CANZ president Beven Hanlon.

"The only reason I am speaking out is because ... I think union members should know and have more input into how their money is spent."

Williams said her concerns about finances included spending on credit cards.

She said she had been on trips where other union officials paying with a union credit card had paid for extravagant meals.

The union credit card was also used for two officials to take a helicopter flight in Australia.

She said union fees had increased recently from $13 a fortnight to $17.50.

Williams said: "We have a lot of money going through the books that is members' money. In the past our union has got pay increases, it has given them good working conditions, it has been a good union.

It is still a good union [but members] need to look for accountability and transparency."

A letter from CANZ auditing firm Sadler Oakly Newman sent in December 2009 warned the union over getting receipts for credit card spending.

Accountant Rod Oakly wrote: "We found during the course of our audit that those holding Visa cards are still not always providing the necessary receipts to the business manager for reconciliation."

The letter also stated "payments are not always being authorised by signing off the payments voucher" and urged that financial information be available to all union members.

Hanlon did not return Herald on Sunday calls.