SYDNEY: Two people have died after a light plane nosedived into a river in front of thousands of horrified spectators at an air show in Western Australia, missing nearby boats and crowds.  

Footage shared on social media showed the Grumman G-73 Mallard seaplane flying above the Swan River in Perth on Thursday afternoon as part of Australia Day celebrations before it went down.

Western Australia Police said the "very experienced" pilot Peter Lynch, 52, and his Indonesian-born passenger and partner, 30-year-old Endah Cakrawati, died in the crash.

"Thank God it went down in the water where it went down -- that was one of my first thoughts," WA Police Acting Commissioner Stephen Brown told reporters in Perth on Friday.

"There's... very clear waterway, that was the clear waterway that Peter was flying over and not too far away on the foreshore are thousands of people.

"And further to the west was quite a large group of members of the public on their boats."

The Swan River runs through Perth city and the seaplane was taking part in an air show ahead of a fireworks display, which had been expected to attract more than 300,000 spectators. Both events were cancelled after the accident.

Ray Johnston was sitting close to the water's edge when he heard the sound of the plane's engines "roaring really loudly".

"It didn't sound right, and when I looked I saw the plane nosediving towards the water," Johnston told The West Australian.

"When I saw the plane it was about three metres from the water and then it hit the water and seemed to split in two. I saw it sink and nothing surfaced."

Brown said investigators did not know yet why the crash occurred.

"We need to know the facts and precisely what happened and why, so we can fix it, if it needs fixing into the future. It may just be a tragic accident," he said.

He added that Indonesian police were notifying Cakrawati's family of her death.

-AFP