A woman's finger was nearly amputated when it became stuck in a boat's boarding ladder in the Hauraki Gulf this afternoon.

Emergency services were called to the 12m launch near Motuihe Island's Ocean Beach about 3.30pm, Coastguard Northern Region said in a statement.

"A woman had got her finger caught in a boarding ladder, resulting in a nasty near-amputation wound," the statement said.

Coastguard vessel Maraetai Rescue One was quickly on the scene with a medic on board.

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The bleeding had been stemmed by the time they arrived but the woman was in severe pain.

She was rushed to the Marine Rescue Centre in Mechanics Bay, where she was met by a St John's ambulance.

Coastguard said it had been a busy day on the water with 18 requests for help and over 1600 trip reports made by boaties since this morning.