Life-savers are guarding popular beaches in Sydney
Shark alarms blared over four Sydney beaches over the weekend, after several sharks were sighted in the area.
Swimmers fled the water and warnings remain in place at the Maroubra, Coogee, Bronte and Tamarama beaches.
No-one was hurt but lifesavers in helicopters reported seeing hammerhead sharks feeding on fish and squid near people swimming.
Some experts believe cleaner water is providing more fish as bait to sharks, bringing more closer to shore.
A swimmer off the west coast of Australia, Brian Guest, was killed by a shark after Christmas.
Officials in Perth, capital of Western Australia, said increased alertness among swimmers also raised the number of reported shark sightings.
Thousands of beachgoers were affected by the weekend shark alarms in Sydney.
Lifesaver Chris Dolman spotted a hammerhead shark, between three and four metres long, during his routine boat patrol at Coogee.
"We tracked the shark for a little bit and noticed it was heading for seven swimmers near Wiley's Baths," he told the Sydney Morning Herald newspaper.
"Once they heard the alarm they did get pretty panicky, as you can imagine."
The shark swam away but was spotted again by a helicopter at Maroubra beach, where it was feeding on a large squid with another hammerhead shark of less than a metre just beyond the breaking waves.
"It was circling a large squid, which was slowly being washed towards the beach," the chief pilot of the rescue helicopter, Peter Yates, said.
Another shark sighting in the afternoon forced swimmers from the water at Bronte and Tamarama beaches.
A New South Wales Department of Environment and Climate Change spokesman, John Dengate, told the newspaper that an improvement in water quality was attracting more sharks.
"As all the pollution controls have got tighter and tighter the water quality has got better," he said.
"There's more fish to eat, which means that things that eat fish like sharks are attracted to the area."
The Australian newspaper quoted big-game fisherman and shark photographer Al McGlashan stating that the population of some shark species, including the great white shark, is on the rise in Australian waters.
He said research groups were hesitant to comment on the alleged increase for fear of sparking hysteria.
In the past 50 years, reports say there have been 60 deaths in Australian waters from shark attacks.