A poisonous Canterbury lake overrun with a toxic, deadly slime has not recovered, and is still subject to a public health warning eight months after it was issued.

With a dry summer ahead, it's not looking good for Lake Forsyth.

The lake near Christchurch has been plagued by cyanobacteria, a toxic alga.

STACY SQUIRES/FAIRFAX NZ Poisonous slime in the lake earlier this year.

A public health warning has been in place since January, the longest period for such a warning at the lake in recent memory.

Sheep, cattle, and at least one dog have died in the lake's water over the years. Swallowing one teaspoon of cyanobacteria could kill a dog or a child.

Algal blooms are a regular problem at the lake and date back about a century, but locals say they have become more severe in recent memory.

ENVIRONMENT CANTERBURY/SUPPLIED How Lake Forsyth used to look.

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The exact source of the blooms is unknown, but is likely a mixture of weather, the shallowness of the water, and the phosphorous-rich, volcanic soil piled a metre deep on the lake bed.

When Forsyth was opened to the sea last week, the thick, green sludge drained through the canal and into the blue ocean water.

KIRK HARGREAVES/FAIRFAX NZ A rotting sheep's head in the lake in 2014.

John Stroh, who lives near the lake, took photos of the canal and said the water was darker than he'd ever seen it before.

When the canal opened to the sea he crossed it in his dinghy to his car in Birdlings Flat, and said he could see the state of the water up close.

​"When you get a bad year like we've just had, with the warm weather and the algae increases, the bacterial count goes up and doesn't go away," he said.

JOHN STROH/SUPPLIED Water being flushed from Lake Forsyth into the ocean.

"I think it's probably now as bad as it's ever been."

He said Forsyth was different from other lakes, in that its problems were mostly natural.

Some in the community were dedicated to improving its health, but the funds required for a permanent fix were too high for groups such as the local runanga.

"It's different in many ways in that it's a natural phenomenon caused by many factors, and very few of them are human. I think we have to acknowledge that," Stroh said.

"The issue is one of centuries, not of decades. There isn't one silver bullet solution to all this."

Possible fixes include dredging the lake or digging a deep channel through its centre, both multi-million dollar projects.

While the lake had become less toxic through the winter, it did not reach a state in which the health warning could be lifted.

It was unusual for an algal bloom to survive the winter, said ECan surface water science manager Dr Tim Davie.

"We would normally expect the colder winter weather to have caused the cyanobacteria to have died back to well below warning levels," he said.

"Trying to predict cyanobacterial blooms is very difficult, so we can't say whether it is likely to get worse soon."

He could not recall a period when the lake had a warning for this length of time.

A permanent health warning was not justified at this stage, he said.