As many as one in two blood donors have not been turning up to their appointments recently, according to the Red Cross, causing a severe shortage of blood.
New South Wales is in need of 2500 donations in the next 10 days, while another 2000 are required in Victoria.
The Red Cross' Erin Lagoudakis said one donation was needed every 18 seconds.
"So we really need people continuously coming," she said.
"If you've only donated once donating once more will make a huge difference."
She said giving blood really was giving the gift of life.
"It is a bag of life that will go to a patient that you will never know or meet but it means that that patient may get a chance at survival.
"They may get to spend Christmas or New ones with their loved ones."
A Melbourne man who gave blood 74 times before one of his donation appointments led to him discovering he needed life-saving surgery is among those urging Victorians to roll up their sleeves.
Regular plasma donor Darryl Deller, a fit 63-year-old, learnt he needed a quadruple heart bypass after an irregular heartbeat was found when he went to donate in Bundoora.
Having had open heart surgery a few months ago, he can't wait to return to saving lives himself.
"It's just great to know that you could be saving lives," he said.
"As soon as I can come back I'll be straight back doing the same thing again, because it's just a great feeling."