Auckland Council has been fined $18,000 after admitting it damaged three volcanic cones.
It has also been ordered to pay $4000 in court costs.
The council appeared before Auckland District Court on charges laid under the Historic Places Act.
In imposing the fines, the court accepted an early guilty plea by the the council and other mitigating factors including an immediate halt to the the work as soon as it became aware of the problem.
A contracting company, Frame Group, was also fined $4500 must pay $500 in costs.
The damage occurred last year and related to fencing work being done on Maungarei (Mt Wellington), Puketapapa (Mt Roskill) and Taurere (Mt Taylor).
The council says it accepted responsibility for damage to archaeological sites and that it had taken steps to prevent an recurrence.
The Historic Places Trust says it never gave authority for the work, which damaged terraces and midden, and exposed artefacts.
The Trust says the sites were listed as archaeological features in the council's district plan, and talks have been held about how to prevent a repetition.