Swimmer is lucky to be alive after being 'rammed' by massive 3.5metre great white shark

 A man has escaped unscathed after being knocked on his side during a terrifying encounter with a monster great white shark at a popular beach.

The swimmer was 'rammed' by the 3.5 metre predator 150 metres offshore from Melros Beach in Perth's south on Sunday afternoon. It's believed the man had dived off a reef in shallow waters before the frightening encounter.

The sighting prompted authorities to alert beachgoers and launch a search for the shark which swam away without causing any harm, much to the man's relief.

Florida Beach (pictured) in Perth's south was closed on Sunday after a swimmer's terrifying encounter with a 3.5 metre shark

'A great white shark has rammed a male person in his side, off the back of the reef at Melros Beach Mandurah,' Surf Life Saving WA tweeted.

'Nil injuries to swimmer, for awareness of other swimmers in the area Melros, south of Mandurah.'

The Department of Fisheries Shark Operation Unit said the encounter took place at nearby Florida Beach, which is a kilometre north of Melros.

'To reduce the threat to people, Florida Beach has now been closed and will remain closed for 24 hours when City of Mandurah will reassess the situation,' an alert read.

The department added it will work with local authorities on a coordinated response and dispatched a Surf Life Saving to monitor the vicinity. 

Locals believed the swimmer's encounter was more likely to  have been with a tiger shark and that sightings in the shallow reef area are uncommon.

'There's not many whites, they're normally out way deep, they come in if there's schools of fish,' a resident told The West Australian.

'They go where the food is. It obviously wasn't interested in the person it bumped, that's not really predatory behaviour.'

It's understood a great white shark (stock photo) 'rammed' the man on his wide before swimming away 

Another added he spots great whites in the area around once a month.

The last fatal shark attack off the Mandurah coast was six years ago when a 29-year-old surfer was mauled at Falcon Beach.

It comes almost three weeks after British expat Simon Nellist was mauled to death by a 4.5 metres great white shark during his daily swim off Little Bay in Sydney's east.

It was the first fatal shark attack in Sydney in almost six decades.

Simon Nellist (pictured with his fiancee Jessie Ho ) was mauled to death by a great white shark in Sydney's east  on February 17