Updated 1:05pm: Two Coromandel Harbour oyster farms are putting off today's harvesting, as efforts are made to minimise the impact of a diesel spill.

A private vessel sank on its mooring overnight in Waipapa Bay off Windy Point.

It had around 400 litres of diesel onboard.

The vessel's owner is arranging salvage, and hopes to have the boat re-floated by this afternoon.

The cause of the sinking is not yet known.

Waikato Regional Council on scene commander Adam Munro says large booms have been set up around the boat to contain the spill.

He says their main priority is to minimise any more spillage and monitor any shore clean up that might need to happen.

The local Harbour Master, Department of Conservation wildlife specialists and regional council oil spill teams are working to contain the spill.