A helicopter crashed into the top of a Hilton hotel in northeastern Australia on Monday, killing the pilot and igniting a fiery explosion on the building's roof.

Hundreds of patrons were evacuated from the DoubleTree by Hilton in the tropical tourist hub of Cairns after the twin engine helicopter crashed around 1:50 am local time.

Mangled pieces of the helicopter's propeller landed in the hotel's pool, an emergency services official said.

Witness Amanda Kay said her assistance dog woke her up before the crash, which she watched from her balcony nearby.

"There was a light chopper and it was flying super low, with no clearance lights on," she told AFP.

"It was flying so erratically.

"There was this huge explosion because it had crashed into the building. It was a big bang."

Police said the pilot, who was the sole passenger in the helicopter, died at the scene.

Two people who were staying in a room close to where the aircraft crashed were hospitalised for smoke inhalation.

- 'Madness, man' -

Images showed a bright plume of fire blazing on the hotel's roof.

"They just flew into that building," a female voice says in a video shared on social media that captured the aftermath.

"Madness, man," she adds as sirens blare in the background.

"Shivers. People were living in that. It smashed right in."

Queensland Ambulance supervisor Caitlin Denning said the aircraft's propellers had "dislodged".

"One landed on the Cairns Esplanade and there was a second propeller located in the hotel pool on the bottom floor and it was on fire," she told local media.

"There were reports of it sounding like a bomb, and seeing the fire and smoke, a lot of the occupants of the hotel were unsure of the situation."

The roof fire was extinguished later in the morning.

Cairns is a popular tourist hub that offers a gateway to Australia's famed Great Barrier Reef.

- Poor visibility -

Australian Transport Safety Bureau commissioner Angus Mitchell said flying conditions were tricky at the time of the crash, with poor visibility and possibly rain.

"Our job now is to look at the facts that we can gather over the next couple of days, and to establish the sequence of events," he told reporters.

"We want to understand... what the helicopter was doing at the time, and the nature of the flight."

A team of government experts from the Australian Transport Safety Bureau have been dispatched to the crash site.

The safety bureau appealed for witnesses to come forward if they had "photos or video footage of the aircraft at any phase of the flight" or if they "heard the helicopter prior to impact".