More than 6,500 people have responded to a call-out from the Red Cross, after it announced just last week that it was urgently in need of O negative blood donations.

O negative is the universal blood type, and is given to patients in emergency situations when their blood type is unknown.

Facts about donating blood: Every week over 25,000 donors are needed around Australia to keep the supply up

Every week over 25,000 donors are needed around Australia to keep the supply up Anyone between 18 and 70 years of age, and feeling healthy, are candidates for blood donation

Anyone between 18 and 70 years of age, and feeling healthy, are candidates for blood donation It takes about 7 to 10 minutes to donate half a litre of blood

It takes about 7 to 10 minutes to donate half a litre of blood O negative red cells are the most rare type of blood

O negative red cells are the most rare type of blood One blood donation can save up to three lives Source: Australian Red Cross Blood Service

But with about 1,000 cancellations a week from regular donors, the blood bank still needs O negative donors to give blood to prevent another dip.

Cold and flu affecting donations

The Red Cross says many donor cancellations are due to people experiencing cold and flu symptoms in the winter months.

James Daly, the medical director of pathology services for the Australian Red Cross Blood Service, is urging healthy people to get to their nearest donation centre to help out.

"The donated red cells only last 42 days, and we have an ongoing demand for group O negative red cells.

"We'd encourage people to continue to book in over the coming weeks and winter months, to ensure we maintain our supplies."

At the Hunter street Donor Centre in Sydney's CBD, Varun Malhotra is one of the people who has responded to the Red Cross call-out.

He expressed excitement about giving his first plasma donation.

Varun Malhotra donates blood at the Red Cross donation centre in Sydney. ( ABC News: Maisie Cohen )

"It's good to see the plasma coming out. Good to know that some people are going to get some benefit from my plasma," he said.

A few chairs down from Varun, Binyamin Lawal is donating blood.

He has been doing so for nine years.

Binyamin Lawal donating blood at the Red Cross centre in Sydney. ( ABC News: Maisie Cohen )

"I feel like it's a good thing to do," he said.

"It doesn't really take any effort on my part and it's a lot of benefit for other people that are in need of it, so I figure why not?" he said.

He's rarely turned away.

"Maybe once or twice just from sickness or generally an injury or something like that, but most of the times it's been OK," he said.

Not everyone who wants to give blood can

Bags of type O blood that have just been donated await collection. ( ABC News: Maisie Cohen )

Some of the most common reasons for people being deferred from giving blood are overseas travel to countries with malaria, cold and flu, low iron levels or if they're taking medications like aspirin.

Gay men are also deferred from giving blood for 12 months after they've engaged in sexual activity.

Freelance journalist Gary Nunn has criticised this policy.

"The fact that I'm gay and British means I can't give blood in Australia," he said.

"I think it is an unfortunate and discriminatory … [the policy] perpetuates the negative myth that gay men can't be trusted with their sexual health, when perhaps the opposite is true.

"It's very much on the radar of many gay people, and they take a big responsibility for their sexual health in a way that other people might not."

He says he understands safety is paramount.

The Red Cross is reviewing the various policies on restrictions around blood donation. ( ABC News: Maisie Cohen )

"I understand that because of the HIV crisis. When that happened, the blood service had to act, and I think a lot of gay people understand that," Nunn said.

"However, now we're in very different times."

Dr Daly says the Red Cross is looking into this policy.

"We're currently reviewing all our sexual activity-based deferrals with an independent expert panel, who are looking at all the evidence behind the risks involved and current sensitivity and testing," he said.

"Hopefully we'll come back with some recommendations in the near future."

He says the priority is to make sure patients are safe.

"Australia has one of the safest blood supplies in the world because of our care, and attention to those aspects of donors and recipients safety," he said.

The UK relaxed its laws in November last year to allow gay men to donate blood three months after their last sexual activity.

Blood donation leads to diagnosis

Michelle Climpson and husband Jamie Deubago hold up signs about blood donation. ( Supplied: Michelle Climpson )

Michelle Climpson overcame a fear of needles to give blood, but a visit to a donation centre in June 2016 uncovered a problem and she was rushed to hospital.

"My haemoglobin was 59, so I wasn't allowed to donate," she said.

"The average woman is about 120 — I was below half.

"After a few more blood tests, the doctor told me I had some sort of blood cancer."

Michelle Climpson in hospital getting treatment for acute lymphoblastic leukaemia.

Ms Climpson was diagnosed with acute lymphoblastic leukaemia and had to undergo treatment straight away.

Instead of giving blood, she was suddenly a recipient.

She is now in remission, but if it wasn't for her visit to a Red Cross blood centre, her future may have looked very different.

"I feel pretty lucky that I did go to donate and I did catch it early, before anything could have happened," she said.