A severe weather warning has been issued for southeast NSW with a cold front forecast to bring heavy rain, large hail, damaging winds and possible flooding.
Thunderstorms are expected across large parts of the state on Thursday, with more severe storms possible in the north and southern inland areas, the Bureau of Meteorology said.
Those storms could bring large hail, heavy rainfall and damaging winds on Thursday afternoon and evening.
The Snowy Mountains and the southwestern slopes could cop six-hourly rainfall totals between 45mm and 60mm on Thursday, with localised falls up to 100mm possible.
"We've seen high rainfall totals of more than 50mm at some of the alpine ski resorts, and unfortunately that does mean that some of that good snow cover is starting to melt," bureau senior meteorologist Jonathan How said.
Heavy falls are also forecast in the state's western slopes, and in western parts of the ACT.
The bureau issued a number of flood watches, warning that melting snow could contribute to river rises.
Wind gusts could peak at about 90km/h over the ranges to the west of the ACT, as well as the eastern Great Dividing Range from Bombala to south of Crookwell, extending north to the Blue Mountains.
High winds will continue on Friday in the state's southeast, including the Illawarra.
A flood watch is in place for catchments in the state's central and southwest regions, with a risk of widespread minor flooding and areas of major flooding from Thursday night through to Friday.
Affected catchments include the Upper Murray, Lachlan and Mitta Mitta rivers.
"Saturated soils in the Central Tablelands and Illawarra bring an increased risk of gusty winds toppling trees and powerlines," the bureau said.
A flood watch is current for areas including Braidwood, Goulburn, Bombala, Tumbarumba, Tumut, Khancoban and Thredbo Top Station, and catchments around the Castlereagh, Bell, Macquarie, Belubula, Bogan and Queanbeyan rivers.
Heavy rain will continue across eastern parts of NSW over Thursday, before showers move towards the state's northeast on Friday as the cold front moves towards the Tasman Sea, Mr How said.
Communities have been urged to continue to monitor the situation and follow updates.
A severe weather warning for damaging winds and heavy rainfall is in place for the Illawarra, Southern Tablelands, the Hunter, South Coast, Central Tablelands, North West Slopes and Plains, Central West Slopes and Plains, South West Slopes, Snowy Mountains and the Riverina.
New South Wales SES southern zone incident controller Barry Griffiths says volunteer crews have been preparing for flooding at Tumut, Gundagai, Tumbarumba and Temora for a week, but conditions were less severe than predicted.
"We've got two high clearance vehicles, a helicopter with rescue capability, and a number of flood rescue resources ... essentially just in case something develops," he told AAP.
"Even though the cold front seems to be reducing in intensity, it still has the possibility to have embedded within it some severe weather and thunderstorms."
An isolated severe storm had the potential to produce flash flooding at one of the small communities in the warning area.
"So that is probably the the main concern we've got for the next little while," Mr Griffiths said.