While police had earlier said 14 people had been injured, Victorian Premier Daniel Andrews confirmed that 19 people had been taken to hospitals.
The police said they have arrested the driver and a second man.
"Having said that, we continue to support this investigation with our counterterrorism command to ensure there isn't that connection and there is no ongoing threat".
"At this stage we believe it was a deliberate act".
It is the second time this year that Melbourne, regularly ranked one of the world's most livable cities, has been traumatized by a vehicle attack. London has seen three such attacks this year, two linked to Islamic extremists and another seen as a reprisal attack outside a mosque, killing 13 people altogether.
Australian Prime Minister Malcolm Turnbull said in a statement his thoughts and prayers were with the victims of the "shocking incident," and pledged federal authorities would work with Victoria's security agencies to investigate.
Police arrived on the scene within 15 seconds and the man was taken into custody by an off-duty police officer after attempting to resist arrest, Patton said. "We haven't identified him linked to any groups", said Patton.
A man who witnessed the auto that hit pedestrians in Melbourne talks about what he saw.
Patton said after reviewing footage police were satisfied only one person was in the SUV, which was registered to a relative of the alleged driver.
"The officer sustained shoulder and hand injuries during the arrest and has been conveyed to hospital", a police spokesman said on Thursday night. He appears to be unconscious and police slap his face and check his pulse before he opens his eyes.
'People were flying everywhere.
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One woman who saw the harrowing incident unfold said she heard screams before she saw "people flying everywhere".
"We could hear this noise, as we looked left, we saw this white auto, it just mowed everybody down", she said.
My sister and her fiance were caught up in the chaos at Flinders Street.
"Then we hear this thump...thump... the auto must've hit the tram stop in the middle".
"Within a minute, the state police were running on foot towards the injured. Standing the tram stop out front of GPO". "And then the armed police were coming down towards us in cars".
A crime scene has been established and all vehicular and pedestrian traffic are advised to avoid the area.
In January, six people were killed and more than 30 were injured when a vehicle was driven up a footpath in Bourke Street, near Thursday's incident.
Sydney, Australia's biggest city, has also installed concrete barricades in main pedestrian thoroughfares.
Witnesses have praised the response from police, who were at the scene on Flinders St within moments of the crash.
Officer Creina O'Grady said police have saturated the CBD area following the incident.
Melbourne Mayor Robert Doyle said two weeks ago that the city faced the threat of an individual inspired by a foreign terrorist organization.