They're back and they're after blood.

In fact, the New Zealand Blood Service is expecting Whangārei folk to again prove they are possibly the country's most generous when it comes to giving up the life juice.

The Blood Service collection team rolled out its paperwork, chairs, collection bags and other equipment in Forum North yesterday morning, opened at midday and had people flowing through the doors immediately.

Within 20 minutes the chairs were all occupied and there was a queue of donors waiting their turn.

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Whangārei consistently shows up as one of New Zealand's highest donor centres, Blood Service collection team leader Scott Sinclair said.

''Whangārei is one of our best collections in New Zealand. We have an awesome response there.''

Sinclair said blood needs fluctuate and during this visit to Whangārei the Blood Service hopes to pull in as many donors as possible with O negative blood type. Nationwide, that's the current push, he said.

Only about nine per cent of the population are ''O negs'' — but they're the donors whose ''universal'' blood type can be given to very sick babies, accident and emergency cases or other vulnerable patients.

All blood types are welcome, though.

Most donated blood is processed into plasma, red cells and platelets, and has a shelf life of 35 days. Large amounts of plasma are needed to make special plasma protein concentrates.

''That's why we need constant donors,'' Sinclair said.

About 470ml of blood is taken during the process, plus 15ml for test samples.

That's about 7- 8 per cent of the blood volume of an average adult (or 1.5 cans of soft drink) and it is replaced by the body within 24 hours.

The Blood Service comes to town about every three months and as well as the ''casuals'' has always had a large number of committed regulars.

Many are getting older now, but the good news is a younger brigade of donors is stepping up to the mark.

The Catholic Women's League volunteers are always on hand to dish out the coffee, tea and biscuits to help top up the donors after their good deed.

The Blood Service is collecting at Forum North from midday to early evening until Wednesday, then at the Cobham Oval cricket pavilion on Thursday and Friday during the same hours.