Three rescue units, five helicopters and multiple fatalities.

The wedding bus lay on its left side with its 35 trapped passengers when emergency services arrived to the carnage late on Sunday night.

Within 45 minutes of the first emergency call, NSW Ambulance commander Luke Wiseman was at the Hunter Valley crash scene.

Nine passengers were already dead.

Another died en route to hospital.

“It was a complex operation,” he told Sydney radio radio on Tuesday.

“Unfortunately, as we all know, not everyone could be released (from the bus).”

Police said bus driver Brett Button lost control of the vehicle as it entered a roundabout in fog about 20 minutes after leaving a wedding at Wandin Estate winery in Lovedale.

Emergency services workers smashed the front windscreen and used the airbags to elevate the bus in a bid to free survivors.

Helicopters were unable to land close by, due to the thickening fog.

“It was challenging – (we were at) an isolated location,” Supt Wiseman said.

“You had multiple people on the bus and as we all know, health (services) is really, really pressured.”

Some of the injured were transported to Sydney hospitals by helicopter.

Acting Police Assistant Commissioner David Waddell said 14 wedding guests aged from 20 to 60 remained in hospital with a range of injuries, while 12 have been released.

“Some of our police are visibly distraught,” he told reporters on Tuesday.

“The initial response was quite chaotic for those attending police and other emergency services.”

Supt Wiseman said one of the first arriving on the scene was a young paramedic undergoing training.

“She had just completed her duty,” he said.

“It was within the first 15 minutes of her drive home that she actually came across the incident.”

Premier Chris Minns visited John Hunter Hospital to speak to emergency department staff who dealt with an influx of trauma patients in the middle of the night and paid tribute to the professionalism and dedication of all the emergency workers.

“It would surprise no-one … that those people were on-site working hard on behalf of the state almost immediately,” he told ABC TV.

“They were doing it with dedication and professionalism, but we need to make sure we’re there for them in the coming weeks.

“This is going to be a traumatic few weeks, perhaps even worse than the initial shock of learning of the disaster on Sunday night.”

The 58-year-old bus driver faces a slew of charges over one of Australia’s worst bus crashes and was granted bail on Tuesday.