Australia's stocks of donor blood have hit critically low levels with less than three days of supply remaining.

The Australian Red Cross Blood Service has a total of just 2.2 days supply of all blood types following a steady decline in donations - less than half the six days of supply it needs.

Since February, the Red Cross has attracted 27,000 fewer appointments than required to maintain its supply levels.

Blood Service spokeswoman Kathy Bowlen said the Red Cross had no explanation for the drop-off.

"People in the business a decade or so are saying they've never seen it this bad in terms of a long, slow decline," she told AAP.