Light plane crashes into the ocean off popular Perth beach

A pilot and their passenger made a lucky escape after a light aircraft plunged into the ocean off popular City Beach in Perth on Saturday.

The incident came after the pilot reported engine troubles during a joy flight and was forced to ditch the small plane.

The two men were able to get out of the aircraft and swim approximately 20 metres to shore following the controlled manoeuvre.

WA Police say neither suffered any serious injuries.

Emergency crews responded to reports of an aircraft crashing into the water around 3.30pm (6.30pm AEDT) with multiple cops cars on the ground, along with Water Police and a helicopter.

Authorities say the aircraft began experiencing difficulties approximately 20-30 minutes after taking off from Jandakot Airport.

"The pilot suggested that there was some engine problems, he had flown north along the coast and he just said to me now, he was looking for a safe spot to land," WA Police Inspector Vic Hussey said.

"So, the safest spot he could work out was the ocean, so that's where he's landed, and its definitely a great job that he's done today.

"Ideally you would land an aircraft on land, but he did a great job getting the plane to where it is and getting out of the aircraft.

"It's a lucky day for them - to be up and around and talking about it".

It's understood the incident was witnessed by a number of people, with the beach busy amid temperatures of 31C in the city.

A handful of bystanders rushed into the surf to help.

"There were people that jumped into the water to help them... I think that's a really brave thing to do," Inspector Hussey said.

"[They] put themselves at risk, as you can see the plane's moving around in the surf as it is, so there's a little bit of risk to say the least to get into the water in those circumstances."

In the hours since the crash, wreckage started washing up on shore and authorities are now in the process of removing the plane from the water.

Meanwhile environmental groups will check for any fuel or gas that has gone into the water.