A mercy dash by the police helicopter crew with blood for the mother of newborn baby is a first for Northland.

The Auckland-based helicopter crew, more used to chasing fleeing drivers or criminals from the air, was called upon to make the emergency delivery of lifesaving blood on Monday.

Whangārei Hospital haematologist Dr Betsie Lombard said the blood was required for a mother who presented with a postpartum haemorrhage.

"She had a history of antibodies that come and go in her blood. She could not have blood that was positive for the antibodies she produced, since the blood would be incompatible," Lombard said.

"We had enough of the blood that would be needed for her immediate care but were concerned that she may need more, so we urgently requested blood from the New Zealand Blood Service. They, with the help of the police were able to send us the units from Auckland."

Fortunately, hospital staff were able to stabilise the mother, but the helicopter crew had already been dispatched.

"Even though this blood was not immediately necessary, it was good to have units available while the patient was still in danger of bleeding again."

Lombard said sometimes specific blood types were essential and were needed urgently.

"In spite of all our systems in place, it is often not possible to predict the amount of blood needed in a specific situation and we then rely on emergency measures.

"We are extremely grateful for the speed in which New Zealand Blood Service and the police responded."

A police spokeswoman said the rescue helicopter and a local charter helicopter were both unavailable and the police team were happy to assist.

The Eagle helicopter crew collected the blood from the Air Support Unit at Mechanics Bay in Auckland about 4.30pm. Thirty-five minutes later, the helicopter had made the delivery to Whangārei Hospital staff.

Police believed it was the first delivery of blood by the police helicopter in Northland.

"On occasions where Eagle has been requested, we have travelled to Northland, as we do for other districts, for a number of incidents over the years, including to assist in multiple search and rescue operations and armed offenders incidents," police said.

"This is not a common request for Eagle. We were not the first port of call to assist with the transport of the blood.

"Other enquiries were made by NZ Blood Service and, as other helicopters such as the Westpac Rescue Helicopter were unavailable, we were more than happy to assist as it was a potentially lifesaving incident."