Three people have been rescued after a 65-foot catamaran sunk in Wellington Harbour.

The catamaran was in the water between State Highway 2 and Somes Island, three kilometres south of Petone, when it started taking on water.

A police spokesperson said they were alerted to the sinking boat at 11.45am. The catamaran is now completely underwater.

The three adults on board were uninjured and reportedly rescued by a passing boat.

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A police launch was also sent out and a helicopter was on standby.

A Maritime New Zealand spokesperson said they were aware of the incident but were not involved in the rescue efforts.

Maritime NZ will be carrying out preliminary inquiries into the sinking.

Coastguard would not comment on the matter.