By Michael Perry

SYDNEY (Reuters Life!) - Sharks, crocodiles, flooding outback rivers and bushfires -- it must be summer in Australia.

Four Sydney beaches including Bondi were closed on Sunday as hammerhead sharks fed on squid close to swimmers, prompting warnings on Monday that shark numbers may be on the rise off Australian cities.

A 51-year-old Australian man was killed by a Great White Shark on December 27 while he was snorkeling off a beach south of Perth in Western Australia.

Helicopter shark patrols now buzz Perth's beaches daily since the killing, copying the summertime aerial patrols over Sydney.

Many shark species, including the Great White, are protected in Australian waters and some wildlife officials believe they are venturing closer to popular beaches.

New South Wales Department of Environment spokesman John Dengate said improved water quality in Sydney, due partly to cleaner ocean sewage outfalls, was attracting more sharks.

"As all the pollution controls have got tighter and tighter the water quality has got better," Dengate told local media.

"There's more fish to eat, which means that things that eat fish like sharks are attracted to the area."

But despite the headlines, there have only been about 60 fatal shark attacks in the past 50 years, according to the Australian Shark Attack File at Sydney's Taronga Zoo.

In the tropical port city of Darwin in the Northern Territory, it's not sharks that swimmers are wary of, but saltwater crocodiles.

A 2.6 meter (8.5 feet) saltwater crocodile has been sighted on Darwin's popular Casuarina Beach in recent days and rangers have been unable to trap it.

"He was just resting on the beach to get some energy. He was probably a bit tired. The water (ocean) is a bit rough," crocodile catcher Tom Nichols told the NT News newspaper.

More than 190 crocodiles were trapped in and around Darwin Harbor in 2008, some as long as 3.8 meters (12.5 feet).

"Nothing changes. The public just have got to be aware this is a croc area," said Nichols.

For Australians holidaying away from the beach in the outback, sharks and crocodiles are the least of their worries.

The outback is dry and dusty for most of the year until the monsoons sweep down from Asia in January and February, flooding dry river beds, which renew pastures but also cut off roads.

Emergency supplies were airlifted to 48 people, including six babies, on Sunday after they became trapped by floodwaters on a highway in the Northern Territory, police said on Monday.

The travelers were stuck on a 38 km (24 mile) stretch of the Barkly Highway for more than 24 hours and police warned outback tourists on Monday to pay attention to road signs as more flooding rain was expected.

Campers in Australia's southern states, such as New South Wales and South Australia, have been warned to extinguish all campfires, as firefighters once again battle bushfires in national parks and farmlands.

(Editing by Miral Fahmy)