The Supreme Court has rejected a bid by Mangawhai ratepayers to challenge their council's right to rate them for unlawful debt.
The High Court ruled in 2013 that the Kaipara Council borrowed $50 million unlawfully to fund the Mangawhai sewerage scheme.
But it could not find that the rates charged to cover those debts were unlawful, because Parliament passed a special bill to validate them.
Mangawhai Ratepayers challenged that ruling in the Court of Appeal and lost.
Their chair Bruce Rogan said now that the Supreme Court has also thrown the case out, ratepayers must cover unlawful council borrowing even when they didn't know it was happening.
The Kaipara Council has welcomed the Supreme Court decision.