A 104km/h wind gust was recorded about 12.30pm at Tibooburra near the borders with Queensland and South Australia.

Heavy rain began to fall around the same time, with 13mm being dumped on the area in just 20 minutes.

Merriwa in the Upper Hunter received almost 16mm in the hour from 1pm to 2pm.

The Bureau of Meteorology warned a "very moist and unstable air mass extends across the state", as a low pressure trough in South Australia combines with a tropical air mass coming from the north.

Senior meteorologist Jane Golding says thunderstorms are expected to develop later on Thursday and a low pressure system will combine to produce "widespread rain and some really intense rainfall rates".

It's likely there will be no reprieve until the weekend with thunderstorms possibly continuing into Friday evening on the mid-north coast and northern rivers.

The northern rivers region is expected to receive torrential rain that the SES warns will increase the risk of flash flooding.

Up to 9am on Thursday, Delungra had recorded the most rain, receiving 94mm, while Narrabri received more rain than it would usually get in the entire month of November, with 76mm.

Ms Golding says the rain is now flowing into the rivers and levels are rising towards flooding, with moderate to major floods possible on the Gwydir and Upper Macintyre River.

However, the flooding is not expected to be as bad as the destructive floods that wrecked havoc in March.

While there is expected to be some riverine flooding, concern is for intense rain from thunderstorms leading to dangerous flash flooding.

The SES received close to 200 calls for help overnight on Wednesday and crews have been sent to flood-prone areas to prepare sandbags, carry out maintenance on flood boats and be on call for assistance.

SES Lismore city unit commander Lacy Loloa says flood rescue techs are on stand-by in case of flash flooding.

SES deputy commissioner Daniel Austin said it was a "really positive outcome" that an empty school bus and three 4WDs caught between two fast-rising creeks near Narrabri on Wednesday afternoon did not try to drive through floodwaters, and encouraged others to follow their example.

Australian Associated Press