An animal expert has revealed why a kangaroo put a hunting dog in a headlock - before it was punched in the face by the dog's owner.

Australian zookeeper Greig Tonkins achieved online fame when footage emerged of him punching the male kangaroo to defend his beloved dog Max, potentially braving a very painful sparring bout with the marsupial.

Dr Mark Eldridge, a research scientist from the Australian Museum, said kangaroos normally viewed dogs and dingoes as predators but that this one may have been surprised by Max and reacted differently, news.com.au reported.

He told the site: "When kangaroos fight they do tend to wrestle and kick, but they would normally view dogs and dingoes as predators and usually flee from them.

"But in this case, maybe the dog surprised the roo and got too close. And in turn, the kangaroo defended itself instead of running away, and did so by getting the dog in a headlock.

"Male kangaroos will try scratch and wrestle before sometimes putting an opponent in a headlock.

"The kangaroo could’ve potentially choked the dog, but it was wearing armour so that may have protected it from the full force."

The video of Mr Tonkins went viral thanks to the quick thinking of Greg Bloom who started to film the incident, which happened during a hunting trip.

Max was wearing armour because the group was looking for wild boar, which have dangerous tusks.

But Dr Eldridge said Mr Tonkins was "very lucky" the kangaroo buck did not strike back, deciding instead to flee.

He told news.com.au: "Before kangaroos fight, they usually size each other up. If one doesn’t back down, they will get into an aggressive encounter.

"I think the kangaroo was still in the sizing up stage when he was hit with the punch, and so the man was lucky the roo decided to call it quits."

Kangaroos have strong arms and a vicious kick.

The hunting trip had been organised for a young man called Kailem Barwick as one of his final wishes. The 19-year-old has since died of cancer.

Mr Tonkins' employer, Taronga Western Plains Zoo, said there was "no suggestion" he would be fired over the punch, but added it was considering "appropriate action".