The Bureau of Meteorology has put the north coast of NSW on alert for severe thunderstorms, with a low pressure trough moving through the region.

BOM meteorologist Melody Sturm said there was the potential for severe thunderstorms in the Northern Tablelands, Mid North Coast and Northern Rivers this afternoon and tomorrow.

"If these thunderstorms develop the main threats are heavy rainfall and potentially damaging wind gusts, as well as a slight chance of large hail," she said.

"In the case of heavy rainfall which we could definitely see flash flooding is a threat in the region."

The BOM has also issued a hazardous surf warning from Tuesday, with large parts of the NSW coastline expected to be exposed to hazardous surf conditions.

"There's a large low pressure system over the southern Tasman Sea, and that is generating a lot of wind that will bring waves coming in our direction," Ms Sturm said.

"That means surf and swell conditions are expected to be hazardous for coastal activities such as rock fishing, boating, and swimming."

The hazardous surf warning includes the Coffs Coast, Macquarie Coast, Hunter Coast, Sydney Coast, Illawarra Coast, Batemans Coast and Eden Coast.