Water New South Wales says spilling out of Sydney’s Warragamba Dam is expected to intensify this weekend with several other major dams also "highly likely" to overflow.
The severity of the spills will depend on what rainfall the catchments receive.
Dam managers are most concerned about four inland dams, including Copeton Dam on the Gwydir River, which was at 98.7 per cent capacity and connects to towns such as Moree in north-west NSW.
Keepit Dam on the Lower Namoi near Gunnedah was at 92.5 per cent and Wyangala Dam on the Lachlan River, which flows through towns such as Forbes, was at 92 per cent.
Further south Burrinjuck Dam on the Murrumbidgee River reached full capacity and was rising Friday evening, with releases to be gradually increased to 25 gigalitres a day.
Burrendong Dam in the north-west was at 113.8 per cent capacity but authorities said it had space to fill flood mitigation areas before spilling into the already flooded Macquarie River.
Sydney's Warragamba Dam is currently spilling at a rate of less than 15 gigalitres a day but that outflow could intensify to up to 275 gigalitres a day by Sunday afternoon.
"Based on the Bureau of Meteorology's [BOM] forecast as it stands, these dams have a high likelihood of spilling to some extent," said Tony Webber from Water NSW.
"As for Warragamba Dam, it's been experiencing a low volume spill since last Thursday of around eight gigalitres a day and has been at, or near, capacity since August last year."
The midweek rain event was not as heavy as predicted for the catchments, which created a window of opportunity for some low-level releases.
"Every bit of extra storage capacity created is valuable to mitigate downstream flooding," Mr Webber said.
The year's 'most significant flooding'
BOM is warning parts of inland and central New South Wales will experience their most widespread and significant flooding of the year so far from Friday.
The outlook is set to worsen throughout the weekend as a trough and cold front moves across the state.
There are 49 flood warnings current with 11 Watch and Act alerts in place, which covers most of the river catchments in NSW.
Additional emergency service crews have been positioned across western NSW — including POLAIR, flood rescue teams and aircraft — to assist with evacuations and resupply operations.
The State Emergency Service (SES) has warned Forbes, Gunnedah and Bathurst are likely to be hit by significant rainfall and experience major flooding throughout the weekend.
Emergency Services Minister Steph Cooke has asked communities to prepare for what is to come.
"We are in the middle of two wet weather systems at present and the next 24 to 36 hours we are likely to see increased rainfall across southern inland and also across Sydney," Ms Cooke said.
"We are expecting weather conditions to deteriorate."
Ms Cooke said the Australian Defence Force will assist the SES by deploying three choppers to be used for night time flood rescues across the Hunter, Greater Sydney and Forbes regions from Saturday night.
Busy roads and poor timing
The warning comes as tens of thousands of people have travelled across the state during school holidays and thousands are expected to travel for the Bathurst 1000, where more than 300 police have been stationed.
"We are pre-positioning resources right across New South Wales to make sure we're able to respond to anything," Ms Cooke said.
Superintendent Bob Noble said the recent drowning death of a five-year-old boy near Dubbo showed just how dangerous floodwaters could be.
"You can imagine how devastating that is for the family," Superintendent Noble said.
"It's a terribly hard way to learn a lesson for society but let's learn the lesson," he said.
The SES has received 365 requests for assistance during the past 24 hours, four of those flood rescues.
The Bureau of Meteorology has warned potential major flooding would likely be worse than flooding those areas have experienced during the past 12 months, with the Gwydir, Namoi, Macquarie, Lachlan, Belubula and Bogan rivers of most concern.
Closer to the coast, that includes the Wollombi Brook, Goulburn and Hunter Rivers.
Many landholders in parts of the state's west around Condobolin have become cut off by floodwaters while part of Lake Cargelligo, north of Griffith, has been told to "prepare to isolate".
Volunteers have begun doorknocking in communities of concern to help them sandbag, relocate or stock up on essentials.
Widespread falls of about five to 50 millimetres have been predicted from today through to Sunday.
There may also be isolated thunderstorms, which could result in heavier falls.
Livestock dying
Richard ‘Tich’ Harries a mixed farmer at Nyngan, north-west of Dubbo, said he was concerned about the forecast.
He said some of his merino sheep had already died and others are suffering.
"They're just eating what food is being dropped to them," Mr Harries said.
A helicopter pilot has given Mr Harries 45 bales of hay so far with more to come.
"We will expect losses probably in the next couple of weeks when you start getting fly in the sheep and their feet start getting sore," he said.
