An image supporting Bernie Sanders' US presidential campaign has been blocked by Facebook in what a technology commentator believes is the first case of political manipulation of automatic moderation.

Australian Facebook user Scott Collins became aware of the situation when a friend in the US asked him to post an image that was being blocked.

"He said Facebook has been taking down Bernie stuff if it's pro-Bernie," Mr Collins told ABC radio.

"For 50 minutes I just kept uploading [the image] and it was security blocked by Facebook — cannot upload this for security reasons."

Mr Collins realised that Facebook was automatically recognising the image and determining that it was not to be posted, so he took action to beat the system.

"I flipped the image left to right so that everything was in reverse, and then uploaded it and everything was fine," he said.

Mr Collins was troubled by what appeared to him to be automated political censorship by the world's largest social media service.

"The reality was that it was very, very difficult and I had to think very hard about how to try and outsmart computer recognition of images and faces and words," he said.

"It was just the most worrying thing because you just think, 'Is this big brother?'"

Manipulating automated image blocking

The pro Bernie Sanders image after it was reversed to avoid being blocked by Facebook's automatic image detection. ( Supplied: Scott Collins )

Technology commentator Trevor Long thinks the image was blocked by the manipulation of Facebook's automatic moderation, possibly by supporters of Mr Sanders' political opponents.

"They are probably definitely blocking this particular image, but I don't think it is because of who he is politically," Mr Long said.

"Because so many people marked this image as 'bad', they're not going to let it go up again because they don't want to have to deal with the complaints."

Mr Long explained that with around 1.5 billion regular users, Facebook had automated systems designed to prevent offensive and illegal images.

"Millions of images are uploaded to Facebook, probably every few minutes, so they have computers that do the basic recognition of what's right and what's wrong," he said.

"The reason Facebook has that function is so things that shouldn't be on Facebook for copyright, or pornography, or abuse reasons, will get [automatically] taken down.

"If 100,000 people say, 'this is bad,' it's probably bad, so you immediately take it away."

First time seen automated moderation in political sphere

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But, according to Mr Long, this is the first instance of Facebook's automated moderation being manipulated for political advantage.

"This is the first time I've seen it put into place in a political sphere," he said.

"Someone at the back end needs to get in touch with Facebook and say, 'I think your computers might be getting tricked here,' and that might allow the image back up."

Co-presenter of the ABC's political analysis program Planet America, Chas Licciardello, said it was surprising the attack was made on the web-savvy Sanders' campaign.

"If it did happen just like we're suspecting, then I think there will be a lot of Sanders supporters who will be annoyed at not thinking of doing it first," he said.

He believes the Facebook manipulation is unlikely to change the outcome of Mr Sanders' battle with Hilary Clinton to become the next Democratic Party presidential candidate.

"Perversely, his campaign is stronger than ever, but I think he's got less chance than ever of actually winning just because of the strange rules of the Democratic primary," he said.

Mr Long said this could be the beginning of a growing problem for Facebook.

"Once this story gets out, there's nothing to stop any group, any party, any couple of people, starting a movement against a particular image," he said.

"I think we'll see people using these artificial intelligence technologies to play the game."