Mangawhai ratepayers have had their first day in the Court of Appeal - in their long-running battle with the Kaipara District Council.
They are challenging a High Court ruling on council borrowing for a sewerage scheme that has left the district deeply in debt.
The High Court found the borrowing was unlawful because the council failed to tell ratepayers what it was doing.
And it said the rates charged to cover the loans would have been unlawful too if parliament had not validated them in a special bill, after court action began.
The Mangawhai ratepayers were arguing that the validation bill over-rode their right to judicial review, and is inconsistent with the Bill of Rights.
They also said ratepayers should not have to carry the can for the banks, when councils borrow unlawfully.