After a man makes racist remarks to a group of people on a Wellington bus, an unknown woman stands up for them when nobody else does.

Police have been asked to investigate a racist rant against two passengers on a Wellington bus after a video of the attack appeared on social media.

NZ Bus, which owns Valley Flyer, confirmed on Thursday it had notified police and had also launched an internal investigation after learning about the incident, which occurred on the Valley Flyer on Wednesday morning.

"We are taking it very seriously. It's the type of behaviour that is not acceptable at all," chief operating officer Tonia Haskell said on Thursday.

A 21-year-old Iraqi woman sitting behind the abusive man took a video on her cellphone of the attack and posted it on social media, prompting widespread outrage.

The woman, who asked for anonymity, said on Thursday she was shocked when the man launched a racist verbal tirade against two young men as they boarded the bus at Petone on Wednesday morning, telling them they should go home and were unwelcome in New Zealand.

The female bus driver growled at him to "cut it out" or he would get kicked off the bus, so he stopped his ranting.

However, after a new bus driver later took the wheel, the man resumed slinging racist slurs against the two men as they got off the bus, which the woman quickly captured on her cellphone.

The footage showed the man calling the two men Islamic who were "shooting innocent people" and telling them to "go back to your own country".

Another female passenger defended them, clapping them and saying they were welcome in New Zealand.

Her intervention prompted the abusive man to start swearing profusely at her and threatening to hit her.

"Shut your mouth up, you b.....," he said numerous times, after she told him to stop speaking to the men like that.

"Idiots like you should be shot," she added.

The Iraqi woman who filmed the exchange said she was terrified to intervene because she was sitting right behind the abusive man and was wearing a Muslim head scarf.

"I was really scared. At first, I thought he was talking to me, then I realised he was talking to the boys, not me.

"I really wanted to say something, but I was really afraid. Imagine what he would say to me. He was even threatening to hit (the other woman) and I thought if I said something, he would get up and punch me in the face."

She posted her video on numerous social media sites as a protest to racism, but was surprised at the outpouring of comments in support of her.

"I would love to see this kind of thing stopped. Noone should judge a whole culture or religion because someone did something bad. We left because our countries didn't have peace and we came here for peace.

"I felt really sorry for the boys."

Haskell said bus drivers had a policy of avoiding confrontation with aggressive passengers because it was safer than risking inflaming the situation, but they were encouraged to contact police.

The abusive man was known to bus drivers as a regular passenger but they were unaware of previous incidents of abuse.

NZ Bus could trespass him from buses if deemed appropriate, but any action would depend on the outcome of its investigation, she said.

"These situations are always more complex than they appear."

Race Relations Commissioner Dame Susan Devoy praised a female passenger who took a stand against the abusive man.

"I'd like to thank the woman who had a go at this man spouting hatred, she is brave and should be congratulated for taking a stand," she said.

"Hate starts small and if we ignore it, then it quickly takes over. My plea is to everyday Kiwis to remain vigilant, to stand up for one another and to not let our country become a place where these kinds of attacks are accepted and ignored. We are better than that."