Australian citizen Saeed Noori was discharged from hospital on Friday and was formally interviewed by police on Saturday.
Twelve people injured in Thursday's car attack remain in hospital, with three people fighting for their lives, Premier Daniel Andrews said on Saturday.
A 32-year-old Heidelberg West man was charged over the attack on Saturday and he was expected to face court in the afternoon, police said.
He was charged with 18 counts of attempted murder and one count of conduct endangering life.
He was discharged from hospital on Friday and taken to the Melbourne Custody Centre.
He remained in custody overnight, as three people fight for their lives in hospital following Thursday's attack.
Noori, a 32-year-old from Heidelberg West is reported to have made "utterances" to police about voices, dreams and the "poor treatment of Muslims" to officers in hospital on Thursday night.
He later made comments about Australia's top security body and Allah.
Twelve people injured in Thursday's car attack remain in hospital, with three people fighting for their lives, Premier Daniel Andrews said on Saturday.
Police say the accused has been taken to a police station for a formal interview on Saturday.
The white car that was driven to run over pedestrians in Flinders Street in Melbourne on 21 December, 2017. (AAP)
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"I think there was something, and I don't know the exact detail, to do with Allah and some ramblings about ASIO (the Australian Security Intelligence Organisation)," Acting Chief Commissioner Shane Patton told News Limited's Herald Sun.
Police Minister Lisa Neville said police had so far found no evidence at his home to suggest he had been radicalised, however, the homicide squad and counterterrorism command are both investigating.
An 83-year-old from Northcote and two South Koreans in their 60s, remain in a critical condition after the 4WD drove on tram tracks and hit pedestrians crossing Flinders Street on Thursday afternoon.
CCTV footage shows the white car pull out of traffic and onto the tracks, accelerating into the crowd of Christmas shoppers before crashing into a bollard.
All up, 18 pedestrians were admitted to hospital, including nine foreign nationals from China, Italy, India, Venezuela, Ireland, South Korea, and New Zealand.
Police raided homes in West Heidelberg and Oak Park on Thursday night. Noori's family members helped police and none were taken into custody.
The accused man came to Australia via a refugee program in 2004.
Mr Patton says the driver was on a mental health plan, but missed a scheduled appointment on Thursday morning.
"The significant flavour of what we're dealing with appears to be mental health, but nonetheless, given what he has said, we continue to explore, fully, all avenues," Mr Patton later told ABC Melbourne.
The driver was arrested after being dragged out of the car by an off-duty police sergeant, who had surgery for a hand injury on Friday.
Noori has yet to be charged over Thursday's attack.
