The assault happened on Saturday about 10.30pm, when the 43-year-old man was understood to have been punched by another man on the forecourt of Mobil Forbury in Hillside Rd.
Detectives and forensic scientists were at the scene on Sunday and the petrol station remained closed and cordoned off until yesterday.
The victim's daughter said her father was a good man who did not deserve the attack.
"He's a good person and a really genuine guy, he didn't deserve it."
The tanker driver, who asked to remain anonymous, said they initially thought the victim was a "drunk person asleep on the forecourt at Mobil Forbury, because you never know what you're going to get there".
"He sounded like he was snoring but he was actually struggling to breathe. I was actually going to go 'hey mate, come on, wake up, get off the forecourt', then I noticed that he had blood coming out of his earsand pooling around the back of his head.
"So I knew he had been struck.
"I ran into the servo and got the guy to ring emergency services, then went out and attended to [the victim].
"The lady on the phone said 'for God's sake don't move him', so I just said, 'Don't move him because he's got a head injury.'"
The driver believed the assault happened after they arrived at the petrol station.
"I was actually around the other side of the tanker, preparing to offload, when I walked around with my paperwork ... I noticed him on the ground then.
"He was really struggling to breathe, I knew he had a massive head injury.
"He was convulsing ... so I knew it was his head."
Ambulance officers arrived shortly afterwards.
The driver said they "just wanted to help".
"You just get peed off when you see somebody get hurt."
