NSW Fire & Rescue Superintendent Paul Johnstone said the damage is widespread.
"This has gone from coast to mountains - all of Sydney has been involved in these storms," he told ABC Radio.
"It's a combination of everything - there's lightning strikes, heavy winds blowing trees and wires down and also the flooding."
Thunderstorms caused major disruptions at Sydney Airport, with 32 departures diverted and more than a dozen flights cancelled.
Extra flights have been added but cancellations and diversions will affect commuters in Sydney, Melbourne and Canberra.
Airport crews had to clear the tarmac because of lightning strikes, while flooding in the baggage collection left some passengers walking around the airport barefoot.
Snow in the Blue Mountains left drivers stranded and an aged care facility without power.
The SES is warning commuters of public transport delays, with the concourse at Parramatta Station in Sydney's inner west heavily flooded.
Flash flooding across the state has prompted at least 57 rescues, with many people being trapped in cars after driving into floodwaters.
Fifty passengers had to be rescued from a train stranded by flooding at North Bexley Train Station in Sydney's south, while six people were helped from a building in Allawah in after a retaining wall collapsed.
Fire and Rescue NSW reported two cases of apparent lightning strikes on houses, one of which caused a large fire at a Kellyville two-storey home, but no one was reported hurt.
The State Emergency Service said that by 4am (AEDT) on Wednesday it had received 1230 requests for help, mainly from the Illawarra, the south coast and south and southwest Sydney.
Sydney Airport was hit by wind gusts of up to 106km/h, but despite reports the airport had shut down, a spokeswoman told AAP late on Tuesday that it was operating.
"The thunderstorms are causing delays," she said.
"Flights are coming in and out, the weather is causing delays to unloading aircrafts ... it will contribute to knock-on delays."
Gusts of 161km/h were recorded at Wattamolla in the Royal National Park, and 115km/h at Kurnell, and high winds were continuing along the coastal fringe of the Illawarra and Sydney early on Wednesday.
NSW's south coast has been hit by the heaviest rainfall, with 170mm falling at Lake Conjola and 145mm at Ulladulla.
Sydney has also been drenched, with falls of 136mm at Sans Souci, 118 at Marrickville, 114 at Peakhurst and 113 at Strathfield.
Up to 15cm of snow was recorded in parts of the Blue Mountains, although the SES said milder conditions and rain had begun thawing the snow by early morning on Wednesday.
A number of cars were damaged by tree branches brought down by snow, and one family had to be rescued after becoming trapped in their car by a fallen tree.
Thousands of homes in Sydney's south have experienced power cuts, including in Bexley, Bankstown and Blakehurst.
The storm has been well covered on Twitter, with disgruntled passengers venting anger about delays at the airport, and NSW residents posting pictures of damage.
- AAP
