Supplies of O-negative blood have reached their lowest point this year, prompting the Australian Red Cross blood service to call for thousands with the rare blood type to donate.

Over the next fortnight, the blood service needs 5000 people to donate, to meet hospital demand.

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Regular donors are still recovering from cold and flu, causing O-negative reserves to fall to less than two days' supply, said Blood Service spokeswoman Helen Walsh.

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"If you are O-negative, someone in your family could be too. We're asking people to talk to their family and encourage them to donate," she said.

A particularly bad flu season this winter has contributed to as many as 1300 donors a day cancelling appointments.

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