Cousins Tevita Mahina and Isaac Tupou, both 17, were on their way to play baseball at Alexandra Gardens. Instead they felt compelled to swing their bats at the driver doing dangerous doughnuts in the middle of one of Melbourne's busiest intersections.

"We just had to take action. No one was doing anything. He looked like he was going to hurt people," Mr Tupou told 7 News.

Mr Mahina said: "The car came at us, and then I stepped to the side, and swung the bat, and then he came back and chased us. He was coming to get us and chased us off."

"I felt bad because I thought we could have done more."

They revealed the driver was yelling "F--k the world" and giving 'the finger' to hundreds of bystanders.

Mr Mahina's father told 7 News that the teens' actions were "very dangerous".

"I said to him: 'What about if the guy had a gun? He might [have] shot you'."

Moments after the cousins tried to stop the agitated driver, he veered onto Swanston Street and allegedly mowed down pedestrians along the Bourke Street Mall.

Five people have died and dozens were injured in the tragic attack.

Donations for the immediate families of those killed can be made via the website www.vic.gov.au/bourkestreet.html

For urgent support, call Lifeline on 13 11 14. Witnesses and victims requiring support can also call the Victim's Support Helpline on 1800 819 817.

A vigil is being planned from 5.30pm on Monday at Federation Square.