A heart surgeon allegedly king hit from behind after telling off a visitor for smoking has suffered a devastating brain injury, a colleague says.

Dr Patrick Pritzwald-Stegmann, who studied medicine in New Zealand before recently moving to Melbourne, is the 41-year-old father of young twin daughters.

Joseph Esmaili, 22, faced Melbourne Magistrates Court on Thursday charged with intentionally causing serious injury before fleeing the scene.

The court heard he had been suffering from drug withdrawals and psychiatric illness, and he did not apply for bail.

Magistrate Suzie Cameron remanded him in custody until August 24 and ordered a medical assessment.

It is alleged the man from Mill Park, in Melbourne's north, had followed Dr Pritzwald-Stegmann into the foyer of Box Hill Hospital and allegedly king hit the surgeon's head from behind on Tuesday night, which was captured on CCTV.

The Royal Australasian College of Surgeons chair of trauma, Dr John Crozier, said Dr Pritzwald-Stegmann, who was knocked unconscious, is likely to suffer serious brain injuries.

Scroll down for video

Heart and lung surgeon Patrick Pritzwald-Stegmann is a father of two young daughters

Dr Patrick Pritzwald-Stegmann (pictured) remains in critical condition after he was king hit outside Box Hill hospital

Melbourne heart surgeon Patrick Pritzwald-Stegmann

'The last information that I had was that the brain injury was devastating,' Dr Crozier told Daily Mail Australia on Thursday

'This is the point of greatest potential in a surgical career. This is a devastating tragedy on so many planes,' he said.

Dr Pritzwald-Stegmann's wife is a Melbourne lawyer.

They are both originally from Germany, but have spent the past few years in New Zealand, before recently moving to Melbourne.

The heart surgeon, who remains in a critical condition at The Alfred Hospital, had reportedly told the man off for smoking in the non-smoking area outside the front entrance.

The attack on Dr Pritzwald-Stegmann is a 'devastating tragedy on so many planes', his colleague Dr John Crozier said

Esmaili was visiting a patient with friends.

His defence lawyer told the court her client was suffering from a number of 'psychiatric illnesses', for which he had been prescribed medication, as well as drug withdrawal symptoms.

'He's currently withdrawing from a drug dependence,' she said.

Prosecutor Ashleigh Harrold said the whole incident was captured on CCTV cameras and identification of the accused will be straightforward.

A married 36-year-old father-of-three from Broadford, in central Victoria, has spoken of how Dr Pritzwald Stegmann saved his life in April 2016.

Dale Brown, who runs a trucking transport business, had a tumour near his lung removed by the talented surgeon, at Melbourne's The Northern Hospital, after experiencing chest pains.

Dale Brown (pictured right with his wife) had a tumour removed by Dr Pritzwald-Stegmann in 2016

'I had no idea that I had the tumour in the chest,' he told Daily Mail Australia.

'I didn't want to have the operation done. I kept trying to put it off and put it off not thinking it was serious and it was him that made me realise how serious it was, get the operation done.'

Mr Brown credits Dr Pritzwald-Stegmann for saving his life and is devastated at the news.

'It just felt sick and angry. The type of guy that he is, all he knows how to do is help, for someone to do that, I just don't get what the world is coming to,' he said.

The attack follows a spate of assaults on Melbourne doctors at hospitals during the past five years.

'Such criminal violence is completely unacceptable for anyone,' Dr Crozier said.

Esmaili was charged with the surgeon's assault following his arrest on Wednesday night after police released a closed-circuit TV image of him getting out of a lift at Box Hill Hospital.

On Wednesday afternoon, police released camera footage of his alleged attacker carrying two plastic bags and wearing a baseball cap.

A CCTV image of the alleged attacker

The front of Box Hill Hospital where Dr Pritzwald-Stegmann was king hit on Tuesday night

Dr Patrick Pritzwald-Stegmann is fighting for his life in hospital after he was punched in the head outside Box Hill Hospital

The alleged assault of Dr Pritzwald-Stegmann occurred at 7.20pm on Tuesday.

'It's believed a man was walking into the Arnold Street premises when he became involved in a verbal dispute with a male standing in the foyer,' police said in a statement.

The man now charged remained at large for a day, prompting a public appeal from police.

Dr Pritzwald-Stegmann worked at several Melbourne hospitals, including Warringal Private Hospital in Melbourne's east.

Ambulance Victoria confirmed paramedics transferred the victim to The Alfred Hospital near Melbourne's CBD.

Victoria's Acting Health Minister Martin Foley condemned the attack as 'completely unacceptable'.

'Our thoughts are with the doctor so brutally attacked at his place of work - as well his family, colleagues and patients,' he tweeted.

The Australian Medical Association's Victorian president Lorraine Baker said attacks on medical staff were sometimes a regular occurrence.

'You should be able to turn up to work and not be abused, threatened or assaulted,' she said.

'Violence and aggression towards healthcare workers must stop'

The surgeon at Box Hill Hospital was reportedly concerned about smoking at the front doors