A 14-year-old boy who stepped in to halt an alleged domestic violence incident in Sydney on Friday has said he "did what any person should do".

Key points: The boy intervened after he saw a man allegedly punching a woman in the face near Riverstone station on Friday afternoon

The boy intervened after he saw a man allegedly punching a woman in the face near Riverstone station on Friday afternoon Police praised his bravery and said he would be formally commended

Police praised his bravery and said he would be formally commended Anyone witnessing domestic violence is advised to contact police

Police have commended the boy, who cannot be identified for legal reasons, for intervening after he saw a woman allegedly being punched in the face by a man near Riverstone railway station in Sydney's north-west about 3:20pm on Friday.

The boy was travelling on a school bus and got off the bus and approached the man, who was at least 100 kilograms and 60 centimetres bigger than the boy.

"I thought, 'That's not right, no-one should think that's right', and no-one else was doing anything about it," he said.

"I did what any person should do and stepped in.

"I said, 'I saw you hitting this woman', and he told me to 'F off' and that it's not my business.

"I said, 'Mate, look, I'm not going anywhere until you leave this girl alone'.

"That's when he grabbed me by the throat."

Bystanders intervened at this point, but the boy said he would do the same again.

"Honestly, I was just doing what any person should do — stepping in. Just being the best I can be, really," he said.

"To be honest, I wasn't fazed by it.

"I just looked him dead in the eye, I wasn't scared of him. In my mind I was thinking, 'If you think you're so tough, you're picking on a woman that's half your size and now you're choking a kid that's half your size'."

The man, 32 from Bidwell, and a woman, 27, left the scene in a red Holden Commodore but police arrested the man a short distance away.

He was charged with assault occasioning actual bodily harm and common assault and was refused bail to appear at Parramatta Local Court on Saturday.

Inspector Jason Shaw from Quakers Hill Police Area Command said the boy would be formally commended.

"It was courageous. For a 14-year-old boy to intervene would have been very, very confronting, let alone an adult getting involved," he said.

"It was very brave and commendable, fantastic work by the young fellow.

"Domestic violence isn't acceptable any time, anywhere, and if you see domestic violence, step up and intervene if you can, and if you can't intervene, ring the police."

NSW Attorney-General Mark Speakman said he was looking for men to follow the boy and "step up" against domestic violence.

"This teenager witnessed domestic violence he immediately knew it was wrong and acted immediately to help," Mr Speakman said.

"I’m not advocating that people get in harm's way but I am advocating that everyone, particularly men, step up.

"What this teenager did gives me hope. It gives me hope that people, particularly men, will step up and speak out against domestic violence."