A new severe thunderstorm warning has been issued for the ACT and surrounding region
Orange, Goulburn, Dubbo, Cobar and Bourke were named along with Canberra in the new warning from the Bureau of Meteorology.
"Widespread thunderstorms are being observed across the state, some of which are likely to be severe," the bureau said in a statement on Sunday afternoon.
Severe thunderstorms were likely to produce damaging winds, heavy rainfall that may lead to flash flooding and large hailstones over the next several hours, it warned.
The State Emergency Service advised residents to secure property, move cars under cover or away from trees, and keep clear of fallen power lines or objects that may be energised, such as fences.
For emergency help in floods and storms, ring the SES (NSW and ACT) on 132 500.
Earlier on Sunday morning, Steve Bernasconi from the weather bureau said the risk of severe thunderstorms had "backed off a little bit this morning", but the bureau did expect there to be more thunderstorms later.
"You should be on the lookout for our severe thunderstorm warnings as the day progresses," he said.
"We have a tropical pressure system that is drawing moist tropical air from the tropics into NSW, that has been creating some unstable conditions in the atmosphere which is leading to wide bands of rain."
The remaining warning from the bureau cautions that heavy rainfall can may lead to flash flooding, as forecast for the South West Slopes and parts of Central Tablelands, Southern Tablelands, Central West Slopes, Riverina, Snowy Mountains and ACT districts on Sunday.
Rainfall in Canberra could reach 120mm, with isolated falls up to 150mm near the alpine peaks.
Temperature-wise the weather is expected to remain warm with 23 degrees forecast for Sunday, with winds north-westerly 15 to 25 km/h turning northerly 15 to 20 km/h in the evening. There is a chance of thunderstorm, possibly severe with damaging winds and large hail and heavy falls leading to flash flooding.
The newly updated map of affected areas now includes Goulburn and regions to the north-east of the ACT.
The bureau described the weather situation as a very moist airmass streaming south ahead of an upper trough will interact from a surface trough moving across NSW.
Embedded thunderstorms could bring scattered heavier falls from Sunday afternoon. Rainfall is expected to ease across the area during Monday morning.
Flood warnings remain in effect for parts of NSW including Orange, Mudgee, Yass, Wellington, Wagga Wagga, Albury, Young, Tumbarumba and Holbrook. The NSW SES advised people to not drive, ride or walk through flood water and keep clear on creeks and storm drains.
NSW Emergency Services Minister Steph Cook said the government was particularly concerned about the heavy rain in Wagga, Albury and Yass due to the risk of flash flooding.
"My message today to communities is, I know that you are flood weary. Our volunteers are also fatigued and wary at this time. I am asking everybody to continue working together," she said.
"We have developed incredibly close relationships with communities right across New South Wales this year. It has been a very difficult year and we are not through it yet. As long as we continue to work together, we will do everything that is possible to make sure our communities are safe, and that people's lives are protected."
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