The attack on Sunday night occurred about 300 metres from where Jill Meagher was raped and murdered in Brunswick in September 2012.

The three bystanders chased and apprehended a 20-year-old Coburg man who will face court on Tuesday charged with intent to commit a sexual offence, sexual assault and unlawful assault.

One of those involved in the citizens' arrest spoke to Fairfax Media but asked not to be identified.

The man said he was riding home from the football along the Upfield bike path in Brunswick at 10.30pm on Sunday when he came across an abandoned bike near Anstey train station.

He heard a woman yelling and stopped.

"Initially I thought it was a couple having an argument so I thought I'd stop and ask 'Is everything OK?'" he said.

The woman allegedly screamed that the man was attempting to rape her as she was wrestled to the ground.

The alleged attacker fled north up Albion Street before crossing west over the train line.

The Good Samaritan gave chase on his bike and was soon joined by two other strangers.

When they caught up with the man at Tinning Street he said the alleged attacker turned and punched him, knocking him off his bike, before the two other men intervened.

"We all tackled him to the ground and he was so strong, it took three [of us] to hold him down," he said.

"This guy was ranting and raving. This guy was not with it," he said.

The witness originally reported it took 15 minutes for police to arrive after they called Triple 0.

A spokeswoman for Victoria Police said officers arrived within four minutes.

She said police arrived with Protective Service Officers on foot after they parked the police car across the other side of the train line.

They then had to wait for a divisional van to arrive.

The woman, a 31-year-old, was assisted by other passers-by back at the scene.

The bystander, a mental health worker, said he couldn't have lived with himself if he didn't stop to help.

"The right thing to do is stop and ask 'Are you OK?' What if that was your daughter or loved one who was attacked?" he said.

"I was the right person at the right time and I had a lot of support from those two guys."

He said he was struggling to comprehend the random nature of the attack, particularly given its proximity to the scene of Jill Meagher's horrific abduction in Hope Street.

Police praised the bystanders for their courage.