Tarrawingee man Brendan Nugent, 43, has been charged with dangerous driving causing serious harm, intentionally causing serious harm, and causing harm to a Commonwealth public official over the incident which left Dr Haines fighting for her life.
It's alleged he intentionally struck the 65-year-old MP with his Toyota LandCruiser outside her electorate office in Wangaratta, northern Victoria, on Monday afternoon.
Dr Haines underwent emergency surgery at The Alfred hospital in Melbourne that night, but one of her legs could not be saved.
She remains in a critical but stable condition.
Ms Haines' husband of 40 years, Phil, and her friend Governor-General Sam Mostyn are due to update reporters in Melbourne later on Wednesday afternoon.
During Nugent's brief court appearance on Wednesday, his lawyer Chirag Patel said the man suffered a rare neurological condition called idiopathic transverse myelitis, caused by a swollen spinal chord, as well as concussion.
He had not being diagnosed with mental health problems, but Mr Patel said he had concerns about his client's psychological wellbeing and wanted him to see a medical practitioner.
Nugent sat quietly in the dock in the packed courtroom, his mouth shut and dark circles around his eyes.
Cultivating and possessing marijuana were among the charges on his rap sheet, court documents show.
Nugent did not apply for bail and was remanded in custody to face court again in February.
Law enforcement have not ruled out further charges, including attempted murder, Victoria Police Commissioner Mike Bush said on Wednesday.
Determining if there was a political motivation for the incident will be investigators' priority, as a joint counter-terrorism task force comprising Victorian, federal police and spy agency ASIO continues to probe the alleged attack.
"His motivation for this, what we allege is a deliberate attack, is very much a centre piece of the investigation," AFP Commissioner Krissy Barrett told reporters in Melbourne on Wednesday.
"Our people will continue to explore every element, angle and avenue to work that out, but at this present point in time I cannot give that information."
That probe will involve trawling Nugent's social media, relationships and personal history, police have said.
Potential connections to the so-called "sovereign citizen" movement, which espouses anti-government ideology, will be examined.
Mr Bush said there was no evidence to suggest connections.
Nugent's former primary school teacher on Wednesday told AAP the accused attacker had been a good kid, but probably had deeper-seated issues.
Jacquie, who preferred not to give her surname, described him as withdrawn, but "beautiful, like most of the kids were".
The Victoria Police commissioner said authorities were also investigating any prior personal personal relationship between Dr Haines and Nugent.
"We're examining social media associations, talking to witnesses, friends and colleagues," Mr Bush said.
The accused's father, Rod Nugent, and Rod's partner Christine Cunningham on Tuesday revealed they had been friends with Dr Haines for more than 20 years, calling the alleged attack "devastating and unfathomable".
Immediately after she was hit by the large off-roader, Dr Haines was still talking and thanking bystanders who intervened, eyewitnesses have reported.
Jacob Evans and Jacklyn Black, who were visiting Wangaratta from Melbourne, rushed to the MP's side on Monday despite not knowing her "from a bar of soap", they told AAP.
"She was thanking us and calling us beautiful people," Ms Black said.
Politicians, who received security briefings from ASIO, federal police and the home affairs department on Wednesday, have mulled over reconfiguring security arrangements for MPs in the wake of the incident.
Prime Minister Anthony Albanese said representatives needed to go about their business, be among their communities and not be afraid of running for public office, urging Australians to disagree with more civility.
The AFP commissioner described Australia's criminal threat environment as "febrile", revealing the agency is arresting an average of nearly one person a week over targeting federal parliamentarians or high office-holders.
