Numerous Good Samaritans tried to stop the runaway, including passers-by who chocked the bus' wheels with whatever they could find, and two off-duty nurses who broke onboard to render first aid.

The bus hit two parked cars and destroyed a power pole - cutting supply to the area - before ploughing through the unoccupied front verandah of a home 200 metres downhill.

The bus was not carrying any passengers; no one was seriously injured in the crashes.

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The Premier Illawarra bus had been travelling westward on an uphill section of Staff Road about 10am on Monday when it crossed to the wrong side of the road.

A resident, Antonia Frino, saw it collide gently with a silver car parked near the top of the hill.

"It was making funny noises. It went all the way up to that silver car and hit it, but very slowly. It was like it was in slow motion," she said.

Several residents saw the driver slumped against the driver's side window.

Witnesses say passing motorists stepped in to chock the bus using rocks, bricks and other found objects.

Mrs Frino later saw two women - off-duty nurses - board the bus after it came to rest against the silver car.

But the women jumped clear soon afterwards, when the bus started moving again - backwards this time - with the driver still on board.

"Just as they must have turned it [the engine] off, the bus started to roll backwards. We were standing here going, 'oh my god, oh my god, oh my god!' and it just kept going," she said.

"My heart was pounding. I was just worried for the bus driver, for anybody that was on the bus.

"It's a busy road and if the bus had kept going straight down, I hate to think what could have happened - it was picking up speed."

The bus smashed into rows of garbage bins, causing trash to rain across the street. It slammed into a power pole, breaking it in two.

The bus changed course - arcing 90 degrees, after it hit a black parked car. It then ploughed backwards through a downstairs front balcony.

NSW Ambulance Illawarra Inspector Norm Rees said the driver, believed aged 62, was found on the floor of the bus, showing signs of having suffered a seizure.

He has been taken to Wollongong Hospital, where he remains in a stable condition. One of the off-duty nurses was also injured.

"When they jumped off the bus as it started to roll, one has sustained a head injury with lacerations, abrasion and contusions."

Emergency services went door-to-door to check on the welfare of all residents in the crash's aftermath.

Fire and Rescue NSW Illawarra duty commander Inspector Greg Purvis said damage to the house was not structural.

"We set up an exclusion zone around the power pole so if it fell it wouldn't impact anything, and assisted ambulance with the patients," he said.

"The frame of the house hasn't been damaged apart from one support on the front verandah which our technical rescue crew will shore up before we remove the bus from the area."

A resident, Donna, praised passers-by who chocked the bus' wheels.

"They were very courageous to see it and try to stop it," she said.

Another resident, Tara, said most could only look on in horror.

"We couldn't do anything," she said. "Everyone just kind of stood. I think the big concern was how hard it would hit."

Premier Illawarra declined to comment on the incident.