Her concerns centred on no person being named as contributor on an electoral return, as required under changes to the act introduced before last year's local government elections.
Mr Brown's return names the trust, but Ms Haden said trustees should have been named and she complained to the Electoral Commission in November, who took the issue to police in February.
Auckland Detective Inspector Chris Cahill said: "An investigation was initiated and is ongoing."
It was reported yesterday that Greg Presland - the main trustee of another entity that paid almost $20,000 in anonymous donations to former Labour leader David Cunliffe during Labour's leadership contest last year - was involved in this trust, too.
Mr Presland, who sits on the Waitakere Ranges Local Board, would not comment last night.
A statement from Mr Brown's office said his campaign complied with legislation and the trust structure was a common practice before the rule change.
"The New Auckland Council Trust made its final donation before the law changed."
