An algal bloom health warning has been placed on Lake Hood for the fourth straight summer, but the lake remains open for now.

Health New Zealand Te Whatu Ora issued the warning on Wednesday after potentially toxic harmful blue-green algae (planktonic cyanobacteria) were found in the lake.

The type of cyanobacteria present in high concentrations is Dolichospermum (formerly called Anabaena) which can be present as cloudy, discoloured water and/or green globules floating in the water column or form surface scums or sheens.

Council infrastructure group manager Neil McCann said the health warning was a surprise given the previous test results, “however, cyanobacteria blooms can appear extremely quickly, in the right conditions”.

A reading for total cyanobacteria biovolume of 10 mm3/L or greater triggers a health warning.

Environment Canterbury testing recorded a 10 at the water ski lane on January 8, with a previous reading at the same location of 1 mm3/L on December 30.

McCann said the lake itself is not closed, but the council is asking people to follow the advice of the health warning.

National Public Health Service Medical Officer of Health, Dr Cheryl Brunton, said the algal bloom can produce toxins harmful to humans and animals.

“People should avoid contact with the water until further notice.

“Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.

“If you experience any of these symptoms, visit your doctor immediately, and please let your doctor know if you have had contact with the lake water.”

Environment Canterbury monitors the lake during summer, one of 97 sites, and the public will be advised of any changes in water quality that are of public health significance.

Last summer, two people reported being poisoned by the algal bloom prior to the health warning being issued on March 20.

In response to the public health risk, the Ashburton District Council closed the lake from April 3 until June 19, while the health warning wasn’t lifted until July 4.

McCann said it was because last summer the toxic cyanobacteria detected was Microcystis and known to be harmful,  and with the two substantiated cases of people falling ill from lake exposure.

“These same conditions are not currently present, so the lake remains open.

“The open status of the lake will be reviewed as required.”

The health warning is unlikely to impact or delay the water quality trials, scheduled to get underway on January 19 ,he said.

We’ll be trialling five algal-control products plus one control container, and also installing several ultrasonic equipment in other spots to see how they can mitigate the growth of cyanobacteria.

Six shipping containers were placed in a canal at the end of 2025 for the first phase of trials.

One container was an untreated control container, and the other five were used for testing three chemical and two biological products.

They are set to return for phase two next week, trailing the same products over a longer period.

McCann said they will also be installing sonic equipment in three other spots in the canals to see how they can mitigate the growth of cyanobacteria.

The weed harvester will also be in operation over the summer.

By Jonathan Leask