Police have confirmed a woman in her 20s, another in her 30s and a 56-year-old died after the bus collided with a four-wheel drive towing a caravan near Gumlu, south of Townsville.

Two men – aged 24 and 23 – are in a critical condition after the Greyhound bus carrying 33 people crashed on Sunday in a 100km/hr zone on the Bruce Highway.

Whitsunday Mayor Ry Collins on Monday said the section of the highway could be challenging.

“The challenge is this is the national highway…if you are to drive the road it doesn’t feel like a highway, it feels like a country road,” Mr Collins told ABC radio.

“There’s no shoulders on the road, it is a fairly open section so I’d imagine the vehicles coming either way would have been able to see each other but there’s obviously been something happened here where one’s veered into the other’s lane.”

Paramedics treated 27 people, with seven initially taken to a Townsville hospital after the crash occurred in the Whitsundays area on Sunday morning.

Four people remain in hospital including the two men in their 20s who are in intensive care in a critical condition.

Two men – aged 27 and 51 – are in a stable condition.

Acting Chief Medical Officer Jason Yates said medical staff acted quickly after notifying the community to only attend the Townsville hospital if their matter was urgent.

“We were expecting a mass casualty event and we needed to be ready to respond,” he told reporters on Monday.

“Our teams worked really hard. We were able to mobilise additional …staff who all came in on a long weekend to do their thing.

“(They) …were able to provide timely care to the patients that needed it both as part of this tragic event but also our regular people that needed our care.”

Police said the bus and 4WD had been travelling in opposite directions near Gumlu when they collided, describing the scene as “very, very confronting”.

The cause of the accident is still being investigated.”An incident of this nature is very complex. We want to go through the holistic investigation before we really move towards looking at how and why it’s occurred,” Superintendent Graeme Paine told ABC TV.

Queensland Premier Steven Miles said he was “shocked and saddened” by news of the crash.

“Our thoughts are with their loved ones and those impacted by this tragedy. Queenslanders are sending you strength,” he posted on social media.

Greyhound Australia said the company was “heartbroken” by news of the crash.

“This is a tragic incident. Our deepest sympathies go out to those involved and their families and friends,” they said in a statement.

The Bruce Highway was reopened on Sunday night, hours after the fatal crash.