Flash flooding, hail and severe winds will lash large parts of NSW over the Christmas period, with residents told to be alert but not alarmed.

The unsettled weather started on Sunday and is expected to run through to Boxing Day, with much of the eastern coastline at risk.

Storm super cells bringing up 150mm of rain and 5cm hail stones are likely, although it’s not yet clear where they will hit.

“It’s always a challenge with severe thunderstorms,” NSW State Emergency Service commander Greg Swindells told reporters on Sunday.

“The best advice I can give for people will be to monitor the Bureau of Meteorology website and also the NSW SES website for any severe thunderstorm warnings.

“They will be issued in a timely fashion and they will give instructions to people on what actions they should take.”

Mr Swindells said isolated flash flooding was likely, but more widespread flooding in the state’s north was not expected.

Lismore in the Northern Rivers region is still recovering close to two years after it was devastated by severe floods.

“It’s natural to be anxious and having anxiety in regards to the weather,” Mr Swindells said.

“I can assure you that the information that we have, we’re not expecting any riverine flooding.

“Our greatest risk here is for localised flash flooding, and also the hail and severe winds that we may see, particularly in the Northern Rivers.”

Mr Swindells advised people across NSW to take care on the roads and not drive through flood waters.

“We’re asking for people to be alert but not alarmed,” he said.

“We want everyone to have a safe Christmas so stay safe, look after yourself and just just be aware of the current situation around you.”

-AAP