Australia's reserves of O-type blood have fallen to a dangerously low two-day supply and people with O-Positive and O-Negative blood types are being urged to roll up their sleeves and donate.

Eight thousand people are needed over the next two weeks to donate O-type blood to prevent a nationwide blood shortage, the Australian Red Cross Blood Service says.

"O-Negative is the universal type given to patients in emergency situations," spokesman Shaun Inguanzo said in a statement.

"And O-Positive is the type that 39 per cent of Australians have - and therefore the type most needed - should they require a blood transfusion."

Maintaining blood supplies during winter is a seasonal challenge because those suffering cold and flu symptoms limit the number of people able to donate.

"With one in three of us needing donated blood in our lifetime, the life you save could be that of a friend or family member."

To donate blood or find out more information call 13 14 95 or visit donateblood.com.au.