The Bureau of Meteorology on Wednesday afternoon warned severe thunderstorms are likely to hit Grafton, Tenterfield, Inverell, Moree Wagga Wagga, and West Wyalong.

"Heavy rainfall is expected to fall in those areas and dealing with very saturated soils to heavy rainfall could lead to flash flooding," meteorologist Melody Sturm told reporters.

Riverine flooding is also possible, she warned, with flood watches issued for the Wilsons, Richmond, Orara, Bellinger and Kalang rivers.

Flooding could reach moderate levels along the Orara, Bellinger and Kalang rivers.

Lismore and Murwillumbah, which were flooded only weeks ago, are in the firing line as well as Grafton, Byron Bay, Ballina, Casino, Kyogle, Yamba and Maclean.

Thunderstorms that have been plaguing Sydney are set to ease over the week, Ms Sturm says.

"For Sydney (the weather) will generally be on the easing trend - some showers possibly, cloudy conditions, but mainly not any thunderstorms."

Australian Associated Press