And those thunderstorms could be severe.

Meteorologist with weatherzone Brett Dutschke said a severe rating could include any one of, flash flooding, large hail or damaging winds.

"You don't need all those features but any one of those three would mean these thunderstorms are rated as severe," he said.

"The most likely outcome for these thunderstorms is flash flooding but damaging winds are also a chance. Large hail would only be a very small chance in these storms.

"It is likely that these conditions will be repeated each day through to Sunday," he said.

Mr Dutschke said the heaviest rain will be on the hinterland areas with the chance of 10 to 20 millimetres of rain on Wednesday.

He said Thursday appears to be the heaviest rainfall day overall.

"The forecasts suggest the hinterland areas are a chance of 20 to 40mm while the coastal strip could see 5 to 10mm," he said.

"The remaining days through to Sunday look relatively less likely for significant rainfall.

"It is not going to be raining all the time either, there will be some dry breaks each day," he added.

Showers and thunderstorms are more likely later in the afternoon and evening.

Despite the showers, it is expected to be warm and more humid than normal for this time of the season.

"This kind of humidity is more typical in later summer," he said. "It could possibly feel up to 3 degrees warmer than the actual temperature."

Persisting north easterly winds, the high humidity and the rain and storms are the result of a slow moving low pressure trough.

The slow moving nature of this system means that there is a good chance of picking up some rainfall each day right through to Sunday.

It is likely this system will still probably linger through to early next week but in a much weaker form.

Wednesday's maximum temperature will be 25 degrees while the range for Thursday is 18 to 29 degrees; 19 to 30 degrees on Friday; 20 to 28 degrees on Saturday and 20 to 27 degrees on Sunday.

Out to sea, the swell will increase to 2 metres by Thursday afternoon and higher again on Friday.

Those conditions should remain in place through to Sunday.

Port Macquarie lifeguard supervisor James Turnham has warned Festival of the Sun-goers to be cautious if entering the water at the northern end of Town Beach during the festival.

"Please avoid swimming beside the breakwall as the conditions are constantly shifting," he said.

"The surf is unpredictable near the river mouth and includes a constant current and rip.

"We will have lifeguards on duty from 7am on Friday, Saturday and Sunday. Flags will go up at the regular 9am start to patrols."

Mr Turnham said the conditions "won't be great"

A small swell from the north east will mean that Town and Flynns beaches will be fairly messy and rippy.

Lighthouse Beach is probably the best bet.

"However, it will be a tricky few days," Mr Turnham added

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