Saeed Noori, a 32-year-old Australian citizen of Afghan descent, is believed to have been unlicensed when he allegedly drove a relative's car into the crowded intersection on Thursday night, leaving 19 people hurt.

On Friday morning Prime Minister Malcolm Turnbull said three people remain in hospital in a critical condition following Thursday's attack in Melbourne's CBD.

"I can confirm 19 people [have been] admitted to hospital. Four were critical, three are now critical. Nine were foreign nationals," he said.

At least one of those injured is from New Zealand, while one of the first named victims was an Indian national.

The Prime Minister said Mr Noori came to Australia as a refugee from Afghanistan through the "normal refugee programs", not via a people-smuggling operation, and had a history drug abuse.

Mr Turnbull said the alleged offender had no known links to any "political issues or extremist groups".

"Therefore, the position I'm advised is that no terrorism link has been established," he said.

But he said a mass of material was being investigated in relation to the attack and "nothing has been ruled out".

"I want to reassure all Australians that this is an isolated incident," Mr Turnbull said.

"We should continue to go about our daily lives in the way we always do"

When it comes to preventing terror, Mr Turnbull said Melbourne has some "special challenges"

"Melbourne has ... big wide streets, wide footpaths and of course, it has trams, and the tramways enable a driver, as this driver did, to get, to pull out of stopped traffic, get into the tramway and then make an attack.

"This is an issue for protecting, for example, the Bourke Street Mall.

"There are a lot of measures that can be done, such as bollards, but it is not possible to do that in every area of Melbourne, so again, that presents some challenges.

"The Premier is very alert to this and we have talked about it. I think it is vital that we ever we can, we build protections into the design.

Driving offences

In June, Mr Noori was convicted and fined $1000 after pleading guilty to driving while unlicensed, using a mobile phone while driving, and failing to answer bail.

The driving offences occurred either late last year or earlier this year.

The charge of failing to answer bail related to him missing a court appearance in May that related to the driving offences.

It is believed to have been his first conviction, after he pleaded guilty but was not convicted of recklessly causing injury in 2010.

He was fined $800, and told his guilty plea had spared him a suspended sentence.

A charge of unlawful assault was dropped.

The nature of the alleged assault is unclear, but Acting Chief Commissioner Shane Patton described it as minor last night.

The car used in Thursday's attack was registered to a family member and police said it was not stolen.

Mr Noori was allegedly sitting about four cars back from traffic lights travelling west on Flinders Street before he drove up on the tram tracks and accelerated into pedestrians. Charges are yet to be laid.

While police believe the motivations of the attack are not terror-related, the driver attributed his actions in part to the treatment of Muslims when investigators spoke to him in hospital.

"He made a number of utterances in respect to dreams and voices, but also attributing his actions relative to concerns for the treatment of Muslims," Mr Patton told Channel Seven's Sunrise program.

"We are obviously going to be exploring the entirety of these matters when set against the backdrop of the circumstances I have outlined to see what the actual cause, the motivation of this, is."

Mr Noori is yet to undergo a psychological assessment but police have made it clear that he has a history of mental illness. He also missed a mental health appointment on Thursday and is a known drug user.

The Homicide Squad, Major Collision Investigation Unit and Counter Terrorism Command are investigating the incident.

Police Association secretary Wayne Gatt said it had been the first day on the force for many of the officers.

"We understand about 16 of the members at the scene were actually on their first day on the job," he said.

"Just imagine that, you're in Victoria Police out in the street after training and being confronted with that sort of carnage, it's a credit to their training.

"We're very proud of our newest members."

Victorian Premier Daniel Andrews described the incident as a "horrific act, an evil act, an act of cowardice perpetrated against innocent bystanders"

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