Australian Prime Minister Malcolm Turnbull is facing a massive backlash over a controversial photo he posted on social media.

It seemed innocent enough ... a photo of the Prime Minister holding his baby granddaughter while watching the footy.

But Malcolm Turnbull's latest Facebook post has prompted a huge backlash online.

The PM has been branded "irresponsible" for the photo, which showed him holding his granddaughter Alice, with a beer in his other hand at the AFL final between the Swans and Bombers at the SCG yesterday.

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He captioned the image "multi-tasking", said news.com.au.

Multitasking at the footy. #goswans Sydney Swans

Posted by Malcolm Turnbull on Saturday, 9 September 2017

But Facebook users were not impressed, calling the PM out for being an "irresponsible" child minder and using the opportunity to criticise him over the upcoming gay marriage plebiscite.

It cames as Turnbull today threw his support behind the "yes" vote in the same-sex marriage debate, telling like-minded Liberals and Nationals it is a question of "fairness".

One Facebook user commented: "Wasting 150 million dollars and watching the footy at the same time. #multi-tasking".

Another wrote: "Does anyone see anything irresponsible with an adult hold(ing) a baby and juggling a beer? And when was drinking while holding a child OK?"

The backlash continued: "I find it disgusting to see people breathing grog all over baby's but sadly I'm not surprised by Malcolm doing it."

Turnbull's Facebook post, which has attracted more than 7000 "likes", was inundated with more than 600 comments - and counting.

While many slammed the PM, others defended the photo, saying there was nothing wrong with it.

One of Turnbull's followers wrote, "To me it's a nice pic of an Aussie Dad at the footy with his grandchild", while another said "good on him for being natural".