WaterNSW's northern NSW region water scientist Tracy Fulford said the blooms at Lake Inverell are "not expected".

"This is a surprising alert as this has come so soon after the floods and was not expected, especially as the days and nights have been getting cooler. But yes, the floods could have contributed to this result by washing in nutrients from the landscape," he said.

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The alert was put in place on Thursday, with the next scheduled water testing set for late tomorrow - the results of which are expected the week after.

"The lake at Inverell often has red alerts through the warmer months when the flows are low or have ceased altogether," Mr Fulford said.

"River flows presently are 32 megalitres a day. This is likely low enough to allow the algae to multiply behind small impoundments along that section of the Macintyre River."

He doesn't expect the algae to persist, however said that it is largely dependent on the river flows and how warm and sunny the days remain.

"People should not enter or come into contact with the water and be mindful of their children and pets that are playing near the water's edge along the park areas along the Macintyre River," Mr Fulford said.

The alert on Pindari Dam has been active since December 8 last year, and is still being sampled weekly.

Mr Fulford said it was very likely that additives to the water from over land flows have contributed to the blooms in the dam.

"[It is] very likely as the widespread rain and overland flows would have washed nutrients into the storage," he said.

"It is not unexpected that the storage would have algae problems after these recent rain events as the storage can act as a sink for nutrients that the algae feed off."

People who believe they may have been affected by blue-green algae are advised to seek medical advice.

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