Auckland oyster farmers are facing another blow just weeks after a major wastewater spill into the Mahurangi River.

Watercare says heavy rain on 19 November caused 86 cubic metres of wastewater and stormwater to overflow into the river from a Warkworth pipeline.

The Ministry for Primary Industries has suspended harvesting while tests are carried out, but growers say the spill has already wrecked their busiest season.

Matakana Oysters owner Tom Walters told Kerre Woodham that, 'it's been a gutting year, and it seems to be the gutting cherry on top of it at the moment.'

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