Almost every river system west of the Great Dividing Range is in flood.
Closer to the coast, residents in western Sydney's Hawkesbury-Nepean catchment have been warned to prepare for forecast flooding at Lower Portland, Sackville, Windsor, Penrith, Colo and North Richmond.
Minor flooding is expected to begin there on Friday and into the weekend.
Severe thunderstorms are set to hit western NSW on Thursday, with a warning area cutting through western NSW from the Queensland border through Mungindi, Moree, Walgett, Coonamble, Nyngan, Cobar and Ivanhoe.
Another storm was set to hit the central tablelands with Mudgee in its path.
"Thunderstorms have been quite hit and miss, but some locations have seen more than 100mm (rainfall)," Bureau of Meteorology senior meteorologist Jonathan How said on Thursday afternoon.
The bureau forecast damaging winds, heavy rain and hailstones with possible flash flooding.
More storms are expected, likely to hit on or west of the Great Dividing Range in northern NSW before moving towards the coast on Friday.
"Another low pressure system is set to form over southern NSW and that will drive further showers and storms into mid-next week," Mr How said.
The bureau warned some areas already affected by floods may be impacted, depending on where the thunderstorms form and the direction they move.
State Emergency Services Deputy Commissioner Ken Murphy said there are serious concerns about thunderstorms hitting the northwest and central west, furthering the threat of floods.
"That is causing some renewed river rises in those areas and some additional concerns," he told ABC TV on Thursday.
The primary flood focus remains on the southern Riverina border town of Moama, where people have been frantically sandbagging and building a 2.5km dirt levee along the banks of the Murray River.
Major flooding began on the river shortly before midnight on Wednesday.
The Murray River is still rising and is approaching the 94.77m height of a 1993 flood, the second-worst on record.
It could reach 95m next week, with more rain expected before then, the bureau warned.
Hundreds have been told to evacuate in and near Moama throughout the week, as well as on the other side of the river at Echuca in Victoria.
Downstream, the Murray River has reached major flood levels at Torrumbarry and Barham.
Australian Defence Force troops are helping threatened communities with sandbagging and doorknocking, working from Deniliquin, north of Moama.
By the weekend, increased river rises are expected to lead to more isolations and significantly more road closures in the area.
The Murrumbidgee River is flooding at Narrandera, with major flooding expected downstream at Hay over the weekend.
Major flooding is occurring on the Darling River at Tilpa in the state's northwest and is likely on the Gwydir River at Yarraman.
Moderate flooding continues at Forbes in the central west, with the Lachlan River likely to peak at Condobolin on Friday.
Flooding continues at Warren, where the Macquarie River has been at major levels for weeks.
The river stopped its slow fall on Thursday and remains above major flood levels, with renewed rises likely.
A cold front on Sunday and another low-pressure system are expected to produce more showers into next week across inland NSW.
Police confirmed a body was found in central west NSW floodwaters at Wallanthery on Wednesday, believed to be that of a 63-year-old man reported missing last week.
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