The Auckland-born Newcastle Knights centre, 22, was driving past a party at a Newcastle house on Saturday night when he saw a group of teens encircling the girl, who was being punched and kicked.
The girl's horrified parents, who arrived in their car moments earlier after receiving a phone call, watched Sau jump from his car and break up the group.
"When Junior arrived, they still had her on the ground, punching and kicking her in the head," the girl's mother told Sydney's Daily Telegraph newspaper.
"But when he came across to see what was going on they all sort of moved back. And to be honest with you, I don't even want to think about what would've happened had he kept driving, because she had already been glassed in the back of the head. (She) remembers someone saying 'we should cut up your face and leave you scarred for life'."
The girl's mother sent a letter of thanks to Sau under the heading: 'Thank you to a very special person'.
She said bottles were being thrown at their car by some of the teens and Sau stayed with them until police arrived.
"This act will never be forgotten by our family," the letter read.
The 102kg, 1.75m Sau said he was driving home after spending the evening at a local bowling club listening to music played by his DJ cousin.
He said anyone would have done the same in his position.
"You see four on one, five on one, beating the crap out of a young girl, that's not fair," he told the paper.
"I just saw the young girl struggling and basically got out of my car to ask the parents if they needed help.
"I used to be a security guard so I guess in the back of your mind you're thinking that you just need to break up the all-in brawl. I hung around for a while too because the kids were drinking and you never knew what else might happen."
Sau has been one of the form players for the fifth-placed Knights this season and in February signed a two-year contract extension reportedly worth $A150,000 per year.
It includes a bonus if he makes the Kiwis, which he hopes to achieve on their end-of-year Four Nations tournament in England and France.
