TWEED Shire Council has ruled out pesticides, sewerage and algal blooms as culprits in a large-scale fish kill at Vintage Lakes in Banora Point.

The council was forced to take action after thousands of fish, as well as some birds and turtles, were left suffocating and floating on top of the water, after excessive weed growth clogged up the Flame Tree Park estate waterway.

Residents reported the fish kill to council last week after a foul smell began to permeate the area.

A Port Dr resident, who did not wish to be named, said he had reported the incident to the council last Saturday.

"Friday night we could smell it. We got up Saturday morning and the whole thing was full of dead fish, turtles and birds," he said.

"I live across the road. The smell is disgusting and the whole lake is covered in a dark, red-orangey weed that's growing all over it."

Botanical Dr resident Bob Ayscough, whose home overlooks the lake, said he had never seen the lake in such poor condition.

"This weed comes and goes, but this time it stayed and has sucked the oxygen out and thousands of fish are dead," Mr Ayscough said.

"The lake is usually alive with birds and fish jumping about, but not now."

Mr Ayscough said the weed had been building up since a drain - believed to be designed as an entrance/exit to the lake for fish - had become blocked.

Council was forced to clean up thousands of dead fish - mainly bull mullet - and dump them at landfill during a series of clean-ups throughout the week.

The council's manager of roads and stormwater Danny Rose said tests had ruled out pesticides, sewerage and algal blooms as the cause of the kill, and the hot, dry weather of late was the likely culprit.

"The reasons for deoxygenation in this part of the lake could include extended warm weather increasing water temperatures in the lake, and a lack of significant rain over summer to flush the drainage system and the decomposition of vegetation in the lake following significant growth since weed harvesting in late November," Mr Rose said.

He said it was rare for these factors to coincide, and the best solution was significant rainfall in catchment areas to flush the lake. He said the orange weed on the surface of the lake was predominantly native red azolla, a water fern which was not responsible for the fish kill.