Dr Haines, 65, was outside her electorate office in Wangaratta, in Victoria's north, when she was struck by a vehicle, sustaining critical injuries, on Monday afternoon.
The independent member for Indi was airlifted to a hospital in Melbourne that evening, where the prime minister has said she underwent surgery overnight but remains in critical condition.
"She remains in a critical condition - I think all Australians will be thinking of her today, and her local community, particularly, will be quite distraught," Prime Minister Anthony Albanese told Nine's Today on Tuesday.
A 43-year-old man is in custody in connection with the alleged attack, which is also being investigated by counterterrorism authorities.
Mr Albanese said on Tuesday that Dr Haines was retrieving something from the boot of her car when the car involved in the attack - a Toyota Landcruiser - made a U-turn before veering off the road and hitting her.
"This wasn't something with a car just going off the off the road, this is something that is very clear was aimed at causing harm to Helen Haines," Mr Albanese told News24.
A knife was seized from inside the vehicle after a bystander performed a citizen's arrest, which Mr Albanese said offered a ray of hope amid the horrific event.
"At this dark and terrible time, a element of light is the fact that a good Samaritan, someone rushed to Helen's defence ... put themselves potentially at risk by doing so, and that person is a hero today," he said.
"At the worst of times, we do see the best of the Australian character, and when it comes to the Australian character, there's no one better than Helen Haines."
The incident is being investigated by Victoria Police's counterterrorism taskforce in conjunction with spy agency ASIO and the Australian Federal Police, with the prime minister saying he will be briefed later today.
Authorities are yet to determine a motive for the attack or establish whether the suspect, from the nearby town of Tarrawingee, is known to Dr Haines.
The prime minister also urged "online sleuths" not to interject debate with unconfirmed rumours while law enforcement did their work.
"What we need is more facts and less speculation at a time like this," he said.
The attack has sent shock waves across the political spectrum, prompted calls to firm up politicians security and outpourings of support from all sides for an MP said to be one of parliament's most well liked and respected figures.
It is the most serious in a string of threats and alleged attacks on politicians in the last year.
"It's very important in a democracy such as ours that we cherish the fact that people can get out and about at community events ... we can't afford to lose that," Mr Albanese said on Tuesday, calling for for the temperature of political discourse to cool.
"I am concerned about division in our society - what unites us is so much more important and so much more powerful than any divisions that we have."
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