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© Nikki Short, The Australian The Clarence River at Grafton at its 8.1m peak this week.
Crippling bushfires followed by an epic flood have punished large swaths of Australia in recent weeks.

Now plagues of disease-infested vermin and insects are threatening a cruel end to a disastrous summer.

Authorities have warned of an explosion in disease, infections and bites because of the perfect storm of high rainfall, fauna dislocation and sewage overflow.

Mosquitoes, rodents, spiders and snakes are the main offenders, while black flies are poised to create a spike in bacterial skin infections and allergic reactions.

NSW Health public health physician Professor David Durrheim said the risks increased as flood waters receded: "On the coast the rain event coincided with high tide and that generally increases the water levels into salt marshes and that's where the salt marsh mosquito breeds."

"Inland, as the water subsides, there will be ideal conditions for fresh water mosquitoes. There will be huge numbers of mosquitoes and the risk will be much greater.

"In areas of poor hygiene domestic flies have also been implicated in bacterial infection."

Professor Durrheim said Ross River fever, Barmah Forest virus and Murray Valley encephalitis were the most dangerous mosquito-borne diseases: "Rodents have also been implicated in outbreaks of a nasty disease called leptospirosis in NSW and southeast Queensland.

"The bacteria can enter the body through cuts and abrasions. We advise people to not enter the flood water, ensure they are covered up and if they suffer a cut they may need a tetanus boost."

The warnings came as hailstones and gale force winds lashed Sydney last night, with the bad weather expected to continue today.

"There's a big line of thunderstorms moving through Sydney at the moment," Bureau of Meteorology forecaster Jane Golding said.

"We expect morning showers and strong winds to continue."

The heavy rainfall caused flash flooding and reports of golf ball-sized hailstones and flooding of roads, particularly in the south of Sydney.

SES spokesman Phil Campbell said about 180 calls for help had been made with 105 from Sydney.

How to keep safe

Mosquitoes: Follow the instructions on repellents; cover the skin; avoid swamplands, marshes and other still water areas; use mosquito coils or plug-in devices at home; and, tip out water receptacles which can act as breeding zones.

Rodents and snakes: Remove waste, especially food scraps; keep doors or windows closed; and, be cautious when cleaning around dark areas behind or under large objects.

Black flies: Wear loose fitting and long sleeved clothing; and, avoid sunrise to mid-morning (10am) and late afternoon (4pm) to sunset times outside.

Spiders: Check pots or garden furniture before bringing them inside; shake clothes before putting them on; and, turn off lights when they're not required.