The Bureau of Meteorology has warned of widespread flooding, with two dozen catchments in danger of minor to major flooding across the state from Wednesday afternoon into the weekend.

Senior meteorologist Jane Golding says some areas could receive a month's worth of rain before the end of the week.

"We're seeing rainfall records broken that we haven't seen in 10 years ... over the course of three days many locations will exceed what they'd usually see in November," Ms Golding said on Wednesday.

"The other feature is that it's being followed by some really cold air, some really gusty winds and some really unseasonable snow to some low levels in the south."

BOM warns moderate to major floods are likely around the Northern Slopes, with the Upper Macintyre, Gwydir, Peel, Namoi, Macquarie, Bell, Lachlan and Belubula rivers likely to flood.

The SES expects low-lying areas near water courses will be flooded. Minor roads and low-level bridges will likely go under as well.

SES Assistant Commissioner Dean Storey said the service had gone into a "heightened state of readiness" and extra resources were being deployed to flood-prone areas.

NSW could see flooding similar to levels in March as the rain would have a significant impact on a large proportion of the state, Mr Storey said.

"The potential impact area for this weather event is quite vast."

He said many roads would be cut off and warned those in flooded areas not to drive through floodwaters.

"Be aware and prepare, stay up to date with your local forecast and warnings, if you live in a flood prone area, know that risk and plan accordingly ... in case you are required to evacuate your home," Mr Storey said.

People in rural areas may also have to move livestock and equipment to higher ground.

Marine Rescue NSW has warned boaters, "especially on inland rivers", to check the bureau's forecast.

"Fast river rises, debris, and swift currents will make boating dangerous so consider deferring your trip," it said.

Strong wind warnings are also in place on the coast for Byron, Coffs, Macquarie, Hunter, Sydney, Illawarra, Batemans and Eden on Wednesday and Thursday.

Storms and floods are expected to be a feature of this summer, more than fires as the bureau monitors a developing La Nina cycle.

Regardless of a La Nina, Ms Golding warned the state would have above-average rainfall this summer.

Areas such as Lismore and surrounds that experienced severe flooding last year should miss the heaviest rains, but should prepare for "some really intense storms ... and dangerous flash flooding".

For large parts of NSW it won't take much to cause flooding because the soil is already saturated from recent rain.

"There's a real flood risk and dangerous flash flooding is on the cards," Ms Golding said.

Australian Associated Press