It's wet, it's wild and it will most likely stay that way in Sydney for most of the week.

The Bureau of Meteorology (BoM) predicts up to 20mm of rain will fall in Sydney today, with warnings of dangerous surf conditions.

Photos have been captured of water spouts off the central coast, with Weatherzone saying the coast from Wollongong to Gosford has had its heaviest rain of the month, receiving between 20mm and 40mm in the past 24 hours.

There is also a strong wind warning for waters between Port Macquarie and Port Hacking, in Sydney's south, with a swell of up to three metres predicted.

Since 9am yesterday, 30.6mm fell at Observatory Hill and 15.4mm at Sydney Airport.

The bureau cancelled an earlier severe thunderstorm warning, which had predicted flash flooding on the northern beaches, as rainfall in and around Sydney eased.

But the NSW State Emergency Service is still warning people to prepare for wet weather in the coming days, and to stay away from flood waters, creeks and drains.

The weather caused problems on the city's rail network, with delays of up to 30 minutes on the North Shore line due to the rain and flow-on delays from mechanical repairs on the Carlingford line.

An electrical storm caused a power surge at Glenfield, creating delays on the Airport, East Hills and South lines.

Transport NSW also warned of major delays for citybound traffic on the M7 between Glenwood and Baulkham Hills because of heavy rain, breakdowns and road works.

Showers, wind and dangerous surf conditions are expected to last until Thursday, before it clears up on Friday, the BoM website said.

Weatherzone predicts the coastal rain will move inland later today and tonight, with light rain expected to fall as far west as Dubbo.

Weatherzone meteorologist Brett Dutschke also said it could be the coldest May in Sydney in more than 40 years.

"A cloudy and wet end to the month will ensure this will be Sydney's coldest May since 1970," Mr Dutschke said.

"Relatively clear skies and dry south-westerly winds have given us many chilly nights."

According to Weatherzone, minimum overnight temperatures averaged less than 10.8 degrees, nearly 1 degree below the long-term average.

The temperature dropped below 10 degrees on 11 nights, when usually there are only eight nights so cold in May.

A cold front dropped snow in Orange on May 12, a rare event so early in the year, Mr Dutschke said.

Weatherzone.com.au is owned by Fairfax Media, publisher of this website.

Follow Environment on Twitter