The New Zealand Defence Force is stepping in to rescue a 64-year-old woman with a fractured leg from Great Barrier Island.

A St John Ambulance crew requested help from NZDF as weather conditions wouldn't allow ambulance staff to transport the patient via helicopter or ferry.

Maritime Component Commander Commodore Jim Gilmour said Offshore Patrol Vessel HMNZS Wellington was sent to the island 100km north-east of central Auckland around midday.

The patient would first be transported to Devonport Naval Base on board the vessel, assisted by a St John paramedic.

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An ambulance would then transport the woman to hospital, Gilmour said.

Gilmour said he hoped the patient would recover quickly.

"We are pleased that we have been able to help and always stand ready to support our communities," he said.

St John's District Operations Manager Doug Gallagher said his team was thankful the NZDF could help out.