YANCHEP Beach was closed this morning as a school of about 50 sharks, some as big as 2.5 metres, took part in a feeding frenzy just 700m off shore.

The beach was closed at 9.40am after pilots on board the Westpac Life Saver Rescue helicopter spotted the sharks feeding on a school of fish.

Department of Fisheries shark research scientist, Rory McAuley said it was a natural phenomenon which had today happened unusually close to shore.

He said the school of sharks were made up of whaler sharks including blacktip whalers, spinner sharks and bronze whalers.

"It's quite unusual to see it so close to shore . . . that really demonstrates to me that we have a remarkably healthy marine ecosystem out there and we're really lucky to be able to see these sorts of things so close to a major city," he said.

"It is spectacular. It's not something I've seen a lot of. I've heard of similar events, mostly outside of the metropolitan region every few years."

"It's a big treat ... it's something we should all be very proud of."

Mr McAuley said the sharks were attracted by tuna, which were feeding on a big school of bait fish.

"They (the sharks) were feeding on a reasonably large school of fish, and there were also some larger predatory fish, probably feeding on the same thing," he said.

"I saw some tuna in there hitting the smaller fish ... and then sharks being attracted by that commotion and coming in for a feed themselves."

"The sharks may have been feeding on the small bait fish as well as some larger predatory fish."

Mr McAuley said beachgoers should not be alarmed by the amount of sharks so close to shore.

"The species of sharks are fish eaters and they're not really a species or of a size that pose any significant threat to human safety," he said.

"People don't need to be overly concerned about such a large number of these sharks off the beach."

Yanchep Beach was reopened at 11.30am.