A traumatised man suffered serious injuries and remains in hospital with multiple broken bones after he was randomly attacked with a hammer while sitting in his car in Papatoetoe, South Auckland.

About 6am on Sunday October 9, the man, who fears for his safety and wishes to remain anonymous, was in his parked car at the Puhinui Mobil station when an assailant approached the driver's side door.

For reasons still unknown, the offender struck the driver's side window repeatedly, breaking the glass, before turning the weapon on the victim, still sitting in his car.

The victim was hit in the head several times by the offender, causing fractures to his skull, eye socket and jaw.

In a state of shock, he put his car into reverse, crashing into the fence of a neighbouring house. But the attacker continued his assault, reaching to grab the victim through the smashed window once the car had stopped.

The badly injured victim eventually escaped from his attacker by driving away. He then drove himself to Middlemore Hospital in a state of shock where he remains eight days later in a stable yet serious condition.

His brother spoke to the Herald (the victim could not due to fractures in his jaw) and said the victim's life has been turned upside down by the attack.

"He can't sleep because of nightmares, but when he does, he is waking up scared asking if he's safe, asking if the man has been caught, asking if the windows and doors are locked."

Damage to the victim's car after the attack. Photo / Supplied

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A worker at the Puhinui petrol station said the attacker initially went to the pay window, speaking incoherently. He then walked over to the victim's car and appeared to ask him a question.

The worker believed the attacker may have been under the influence of drugs, alcohol or both.

After a brief moment, the man pulled a hammer from a backpack he was wearing and began his assault.

A police spokesperson said the attack was reported at 5.50am that morning. The complaint alleged an offender had smashed the driver's side window of a vehicle and assaulted the victim.

Police are investigating the attack and making inquiries to identify the offender.