Dams and rivers are overflowing across the state after a month of heavy rainfall.

State Emergency Services volunteers were called upon to rescue 12 people from floodwaters overnight, with 120 rescues performed since the heavy rain set in.

Premier Dominic Perrottet said that number was too high.

Driving through flood waters "puts the lives of those flood rescue teams at unnecessary risk, it is also putting your own life at risk", Emergency Services Minister David Elliott added.

The SES received 300 requests for assistance in the 24 hours to Sunday morning, Commissioner Carlene York said.

She said the weather had been a "significant event" across the state.

"All our dams are full and some are on white alert, which means they're going up to capacity," she said.

"The rivers are full and the ground is quite saturated."

In the central west, the Namoi and Lachlan rivers have posed the biggest risk.

On the Namoi, Gunnedah is experiencing major flooding as levels peak. There's also major flooding at Narrabri and Wee Waa, which will be cut off for about a week.

The Lachlan River's main flood peak is approaching Forbes, where levels are set to peak on Monday afternoon. Jemalong is also seeing major flooding.

A caravan park at Wagga has had to be cleared as well, with residents moved to higher ground.

In the Upper Hunter, residents around Singleton were ordered to evacuate on Saturday evening.

While the rain has eased, another front is threatening to bring further downpours later this week.

Ms York said some of the flood rescues were people driving into floodwaters, but others were people who'd become isolated between floodwaters.

"If you're in an area of high risk of flooding, please just do essential travel and don't go out unless you really have to," she said.

Australian Associated Press