The moment a woman racially abused an Asian passenger on a Sydney train has been caught on camera, as stunned commuters watched on during a minute-long rant.

The verbal attack occurred about 9:30pm on a service between Chatswood and Epping on May 11.

A witness claimed a dispute about littering sparked the incident. ( Supplied )

The video was filmed on a mobile phone by another passenger, who provided it to ABC News on the condition of anonymity.

"I feel bad I couldn't help the girl but I am also Asian and I was worried I would also be attacked if I stepped in," they said.

"I hope the video can make people aware of this issue and stop racism in Australia.

"There were about 15 people on the carriage but no-one did anything."

They said the incident was sparked after the alleged victim noticed the abuser littering, and asked her to clean up the mess.

Trains boss condemns attack

Sydney Trains chief executive Howard Collins said there were 1.3 million journeys per day on the city's trains "but one incident like this is too much".

"The important thing I want every single one of my customers to know is if you see something like that, tell us straight away," he said.

"The sooner we understand what's going on the quicker we can get our police involved and deal with such people."

Dr Tim Soutphommasane urged people to speak up if they witnessed racial abuse.

Australia's racial discrimination commissioner Dr Tim Soutphommasane said incidents like this were more common than you might think.

"If a perpetrator of a racist act has done so because of mental illness, you would hope that their wellbeing is looked after," he said.

"But it doesn't take away from the harm that's done from someone on the receiving end of racism."

He described the video as "the tip of an iceberg" and argued most incidents of racial abuse were not caught on camera.