An investigation's underway into a trust that helped bankroll Auckland mayor Len Brown's election campaigns.

Private investigator Grace Haden is accusing the trust of filling illegal anonymous donations.

The anti-corruption campaigner has filed a complaint to the Electoral Commission, which has been referred to police.

But the mayor's office says the donations were made before local government reforms came into effect last year.

Grace Haden has worked for rates-refuser Penny Bright, and has been critical of the council's rates policies.