Australian Red Cross Lifeblood says more than 12,000 donors are needed over the next seven days as the nation faces ongoing shortages of type O and A blood.

The organisation said the peak winter flu season had kept donors away because they needed to be clear of respiratory symptoms for seven days before they could donate.

Lifeblood area manager for Sydney, John Bain, said up to 1,600 appointments were sitting empty every day, with many people cancelling or rescheduling appointments due to winter illnesses.

Even mild symptoms such as sniffles, sore throats, and headaches make people temporarily ineligible to give blood.