The Bureau of Meteorology has issued a thunderstorm warning for residents in the Central West, Riverina and Western districts of New South Wales.
Locations that may be affected include Nyngan, Cobar, Bourke, Wilcannia, White Cliffs and Ivanhoe in Central West and Western NSW.
Residents in Dandaloo, Mudall, Nyngan, Mulgawarrina and Gongolgon have been advised to 'Watch and Act.'
NSW State Emergency Service (SES) said major flooding is occurring at Mudall, and the Bogan River at Nyngan is near its peak.
The major flooding continues along the lower Bogan Rover at Mulgawarrina and Gongolgon.
"Further rainfall is forecast over the Bogan River catchment during the remainder of Monday and Tuesday, which may cause additional river level rises and areas of renewed flooding," NSW SES said.
The Bureau said severe thunderstorms combined with heavy rainfall could lead to flash flooding in Wagga Wagga, Tumbarumba, Selwyn, Tumut, Holbrook and Lockhart in South West, Riverina and Snowy Mountains districts.
Residents have been advised to avoid the Oura Beach Camping Area near Wagga Wagga.
On Tuesday, Sydney has a 70 per cent chance of rain and a possible thunderstorm.
The bureau expects snow fall over the Southern Alps on Tuesday.
In Victoria, an emergency warning - not safe to return- remains current for Barmah and Lower Moira.
"Dangerous floodwater and widespread and significantly contaminated floodwater remain in several parts of the town," VIC SES said.
The Bureau predicts a 95 per cent chance of showers, a possible thunderstorm and likely small hail in the later afternoon and evening ahead of the Melbourne Cup on Tuesday.
A moderate flood warning has been issued for the Goulburn River Lake Eildon and Seymour.
Keep up to date with the latest forecast from the
Follow the latest changes by checking the
If you are in a life-threatening emergency, call Triple Zero (000). Call the NSW SES on 132 500 and Victoria Emergency Services on 1800 226 226 if you have experienced damage from storms, wind, hail or a fallen tree and if a tree branch is threatening your property or a person's safety.
To access this information in other languages, call the Translating and Interpreting Service on 131 450 (freecall) and ask them to call VicEmergency Hotline.
If you are deaf, hard of hearing, or have a speech/communication impairment contact National Relay Service on 1800 555 677 and ask them to call the VicEmergency Hotline
