NSW floods expected to strand 1800 people

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Rising floodwaters are expected to leave 1800 residents in a northern NSW town stranded for five days as emergency crews prepare to dispatch supplies by helicopter.

Heavy rainfall across NSW has already claimed the life of a three-year-old boy, who drowned early on Saturday afternoon (AEDT) when he was swept into a stormwater drain at Bingara.

By early Sunday morning, the Namoi River is expected to reach 6.7 metres, which the State Emergency Service (SES) says will leave 1800 residents at Wee Waa stranded until Thursday.

"This is a very slow flood. Because it's so flat, it stays around for quite a long time," SES spokesman Phil Campbell told AAP.

No one had been evacuated from the area on Saturday, but Mr Campbell said the SES aircraft crews would change their focus from rescue operations to supplying people with food and essentials.

At Moree, about 90km northeast of Wee Waa, 400 residents have been warned they may be asked to evacuate on Sunday night as swollen rivers gradually inundate the floodplains.

About 50 people are already isolated on properties around Moree.

The weather bureau is predicting that Gwydir River tributaries, at Mehi River and Yarraman Bridge, will peak at 10 metres on Sunday night, the highest level since January 2001.

"They're now assessing what effect that will have and whether that will require a full evacuation," Mr Campbell said.

Heavy rain hit the state for 24 hours into the early afternoon on Saturday, also stranding 400 people southwest of Coffs Harbour, on the north coast.

Since Thursday, the SES has rescued 10 people trapped in cars as they attempted to drive through floodwaters, with eight of the call-outs in the state's north.

A helicopter was used to rescue a husband and wife who had become bogged in a tractor near Moree and were forced to spend the night outside.

Moderate flooding is also expected at Tamworth on the Peel River, and flood warnings are also in place for the Macintyre, Macleay and Bellinger rivers.

A severe weather warning was cancelled early on Saturday afternoon.

Meanwhile, on the NSW South Coast, an evacuation warning has been downgraded for residents of 18 properties below Jerrara dam, inland of Kiama, where there were concerns the dam might overflow if there was any more rain.

A ceasing of rain has seen the alert level moved from amber to a less severe white level.

The SES received around 720 calls for help since heavy rains began falling across the state on Friday, with 400 volunteers on hand to help with leaky roofs and fallen branches.