Severe thunderstorms ripped across New South Wales overnight, with thousands of lightning bolts cracking across the sky and hail pelting parts of Sydney's west.

A 37-year-old man died after he was struck by a gas bottle in Sydney's CBD.

Police said the incident took place at The Rocks just before midnight.

He was taken to a nearby hotel by witnesses with severe injuries where police performed CPR.

The man was later rushed to St Vincent's Hospital but died there from his injuries.

Ausgrid reported "widespread power outages and damage" in Sydney and warned people to stay at least eight metres away from any fallen powerlines.

Damaging winds of up to 101 kilometres per hour were recorded at Richmond, while 12 millimetres of rain fell over Orange in just 10 minutes.

Hailstones the size of golf balls also fell near Mittagong and Wollongong.

Social media users shared dramatic video of the storms online.

Wollongong locals also took to Twitter to share photos and videos in the aftermath, with one saying the rain sounded "like cannon fire".

A severe thunderstorm warning was still in place at 1:29am on Wednesday for people in the Hunter and parts of Mid North Coast, Central Tablelands, North West Slopes and Plains, Central West Slopes and Plains and Northern Tablelands forecast districts.

Residents in Greater Newcastle and parts of Maitland/Cessnock areas were also warned at 1:31am to expect damaging winds, heavy rainfall and large hailstones.

But as the thunderstorms were fast moving, rainfall totals were quite low across the state.

VICTORIA

Severe thunderstorm warnings were issued for Melbourne's inner east, south east, northern east and outer east areas on Tuesday night, but were cancelled by 9pm.

Forecasters had warned "very intense and violent" storms could be felt in many regions, including supercells which are some of the largest and most dangerous types of storms and can cause considerable damage.

Earlier on Tuesday, around 2pm, the weather bureau issued a severe weather warning as dangerous storms were detected in the city's north, moving through the south east. Moorabbin Airport saw around 13mm of rain in just three minutes as the storm passed through.

By 4pm, flooding was reported in areas including near Cranbourne West Primary School and on the Princes Highway at the major thoroughfare of Springvale Rd in Mulgrave.

The storm warning affecting the city had ended by 4.30pm.

THE REST OF THE COUNTRY

In the Top End, a tropical low continues to form that is looking likely to become a cyclone dumping possibly up to 300mm of rain in the next week.

Sky News Weather channel meteorologist Tom Saunders said storms were likely on Tuesday.

"On Tuesday there is a high chance of severe storms for central and northern New South Wales," he said.

"We're expecting maybe even supercells which bring a high threat of damaging winds, flash flooding and large hail."

Supercells are "organised thunderstorms" that are larger and higher than usual systems and can also rotate. They can travel long distances and last for many hours so can cause considerable damage.

Check the BOM national warnings summary for the current warnings in your area here.

- With wires