Australians are being urged to roll up their sleeves and donate blood, especially those with the O-negative blood type, as the nation's supply is critically low.

The Australian Red Cross Blood Service says it has just two days' worth of O-negative blood - the universal type given to patients when their blood type is unknown - left in its reserve.

Spokesman Shaun Inguanzo says up to 1000 donors are cancelling their appointments every day because of cold and flu symptoms, placing further strain on existing donors.