A mother has reportedly been sanctioned by a Sydney rugby club after running onto the field and getting involved in a fight in an under-20s match on Saturday.

The incident occurred during the match between Eastwood and Warringah at Epping Boys High School. 

There were 10 minutes left in the match when two players pushed each other and a scuffle resulted.

The mother of an Eastwood player ran on and jumped in 'to defend her son and pushed a player from Warringah', according to 2GB.

The mum has been sanctioned over the incident, which was caught on video, and has also been cautioned by the Sydney Rugby Union, according to the radio station.

An eyewitness to the incident, named Kylie, also gave her account of the mum's actions.

'Two players going at it in back play. The game had moved further down the field and the mum ran on saying, "Get off my son!"' she told 2GB's Ben Fordham.

'All the Warringah players on the sideline started laughing because it was right in front of their bench, so as she was walking off, they copped a spray.

'It was pretty embarrassing ... they're adults, it's not the under-10s.

'It's embarrassing for him [the son] and her, and it's embarrassing for the club. 

'If it was a dad running on, it would be a whole different conversation, no one would think it was cute.

'She took her seat and refused to leave the sideline, and refused to give her name to the officials.'

World Cup-winning former Wallabies captain Nick Farr-Jones backed the woman and praised her for sticking up for her son.

'I love passionate and protective mothers, so good on her,' Farr-Jones said. 

'It would have been spontaneous, no doubt, there wouldn't have been a lot of premeditation.

'She [the mum] is in a spot of bother but just give her a speeding fine, one demerit point.

'If my wife Angie jumped onto the field to protect one of my sons, good on her, have a go.'