Mr Barrett was walking to work through Surry Hills before 9am on Tuesday when he saw a woman being repeatedly punched in the head in a random and unprovoked attack.

"It wasn't a fight, it was an attack and I kind of realised this person was defenceless and just tried to help as any decent human would," the 33-year-old London man told reporters on Wednesday.

The victim, who's 30 weeks pregnant, was allegedly attacked by a 28-year-old Mount Druitt woman she'd never met.

The 26-year-old endured punches and had her hair pulled before she was pushed onto the road. That's when Mr Barrett rescued her by putting himself in harm's way.

"I kind of went to pick her up and she was on the floor holding her stomach and saying she was pregnant," he said.

An off-duty probationary constable subsequently restrained the assailant and arrested her.

Despite bravely intervening, Mr Barrett doesn't consider himself a hero and is kicking himself for not being quicker.

"I took two seconds too long to assess it, so by that point, there were a few extra punches thrown," he said on Wednesday.

"Two or three punches there that probably shouldn't have landed. I should have been quicker."

The London man said the victim received "a few big hits to the left side of her face" as she covered her stomach.

The mother-to-be was taken to Sydney's Royal Prince Alfred Hospital but was soon discharged. She suffered minor injuries while her unborn child was not harmed.

NSW Police say Mr Barrett stopped what could have been a prolonged attack.

"If it wasn't for his intervention ... who knows where it could have been," Detective Superintendent Gavin Wood said.

"We use the word hero at times very loosely. He is a hero and I note that he's downplaying it, but it shouldn't be downplayed."

The alleged attacker is behind bars after being charged with assault. She's due to face court on June 11.

Australian Associated Press