A homeless man who rammed a shopping trolley at a knife-wielding attacker who was threatening police is being hailed and rewarded for his actions, but he insists he isn't a hero.

Michael Rogers emerged from a crowd of onlookers during the attack Friday afternoon in downtown Melbourne, Australia, in which one person was fatally stabbed and two others wounded.

Social media users have dubbed Rogers "Trolleyman" and an online fundraiser for him by registered charity Melbourne Homeless Collective had raised more than $72,000 ($100,000 AUD) by Monday morning.

"Our hero is humble as can be and had no idea about this fundraiser," the GoFundMe page reads. "He is amazing. "We believe his efforts deserve a reward that can really help him out."

In interviews with Australian media, Rogers, 46, has insisted he's no hero. "I threw the trolley straight at him, and I got him. I didn't quite get him down, though. I'm no hero," he told Channel Seven.

He also told Melbourne's Age newspaper he had been on the wrong side of the law himself. The paper reported he had been "in and out of jail" for some 20 years, including a five-year sentence for aggravated burglary, and that he'd had a long history of drug use.

"I haven't had good experiences with police," Rogers told the paper, adding his move to help on Friday was a "spur of the moment" decision.

Somali-born Australian Hassain Khalif Shire Ali, 30, stabbed three men on the street in Friday's attack, killing a well-known restaurateur and wounding two other men.

Sisto Malaspina, 74, died a short distance from the popular Pellegrini's Espresso Bar he had run for more than 40 years. The other two men are recovering in a hospital from non-life-threatening injuries.

Victoria state Police Commissioner Graham Ashton said Shire Ali had also made an "unsophisticated" plan for his vehicle to explode to cause many more fatalities. He had placed several barbecue gas canisters in his pickup, but they failed to ignite.

Shire Ali was known to federal police and his Australian passport was cancelled in 2015 out of concern he planned to travel to Syria to fight with the Islamic State group.

While Rogers won praise from the community, senior Victorian police officials were divided in their reaction.

Deputy Commissioner Shane Patton said Rogers' help was appreciated by police on the scene. "There's no doubt he acted bravely," he told ABC radio. "His assistance was greatly appreciated."

But later Patton's superior, Chief Commissioner Graham Ashton said while Rogers' actions could have led to a tragic outcome.

"I don't like to criticize people in that situation, he's acting instinctively about what he's looking at in front of him," Ashton told Melbourne radio 3AW.

"But certainly if a trolley had hit a police member and knocked him over and then this offender was on top of him, it could have had a tragic consequence. Luckily in this case, it didn't."