The two snakes were spotted and photographed by Dennis and Jeni Whitehead at Kalaru, on the NSW South Coast.

"I've seen plenty of snakes in my life," Dennis said. "When I lived in Sydney a friend of mine had a reptile pit and a snake pit. We used to go to Botany and look for them under sheets of corrugated iron to take them home in a sack.

"This was very unusual though. I know black snakes will rear up at you like a cobra, but I've never seen any like that before."

Dennis said they were driving along the Tathra Rd towards Bega about 11am Monday when Jeni spotted the snakes by the side of the road.

"We pulled up as close as we could without scaring them off and Jeni took the photos out of the window - we weren't going to push our luck too far!"

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Stephen Mahony from the Australian Museum said the photos depict two males fighting.

"They do this, as the pictures slow, by trying to press the other male to the ground. Not by biting as some might expect of venomous animals," Mr Mahony said.

"The reason for the fighting is the secure mating opportunities. This could be through better territories or if two males following the scent of the same female encounter each other."

For more on the characteristics and behaviour of red-bellied black snakes, visit the Australian Museum here