The Red Cross is encouraging more people to roll up their sleeves in winter, as blood bank supplies drop to dangerous lows.

“In late May, we came close to having less than two days' supply of two of our blood types: O positive and O negative,” Australian Red Cross Blood service spokesperson Rebecca DiGirolamo told reporters on Friday.

“About 40 per cent of Australians have O positive blood, and O negative is universal blood type that can be used in emergency situations for any blood type,” she added.

Winter months see many regular donors go down with colds and flu, leading to an increase in cancellations.

“During the winter period we have thousands of people call and they have to cancel their appointments because they have cold and flu symptoms,” Ms DiGirolamo said.

“They're too unwell to give blood, and that certainly has a significant impact on our blood supply. As a result we have to come out and appeal to the public to donate blood.”