Work has finished on a required upgrade of the controversial Kawakawa wastewater treatment plant in the Far North.

The sewage treatment plant was identified in a Northland Regional Council (NRC) commissioned report as one of four possible sources of contamination of the Waikare Inlet oyster farms.

The farms have been closed since late 2001, when a virus was discovered in the oysters.

The Far North District Council said all work required by the NRC to upgrade the Kawakawa system was completed by the two deadlines imposed, December 2002 for Stage 1 and May 31, 2003 for Stage 2.

Stage 2 of the upgrade required the conversion of a pond to a treatment wetland, to treat effluent from the disinfection system and take overflows from another pond when buffer storage capacity was exceeded.

The NRC and Northland Health inspected the plant early this week.

However, a recent NRC decision required the district council to undertake further works to reduce flows to the plant during high rainfall periods.

That work must be completed over the next three years.

The NRC report cited other possible causes of harbour contamination as the Opua Marina, boat discharges and residential properties at Okiato Point.

The Okiato Point residential wastewater upgrade was completed voluntarily in March 2003.

At Opua Marina, a new pump-out system had replaced the previous one.