Ultrasonic technology is being trialled on an Auckland water supply lake.

Sound waves are being used to kill algae at a water-supply lake in what is believed to be a first for the country.

Algal blooms in summer can render water in the Lower Nihotupu Dam in west Auckland expensive to process or unusable.

But a trial of ultrasonic technology, monitored and constantly adjusted by LG Sonic in the Netherlands, has shown promising results in keeping good water flowing from the dam.

SIMON SMITH/FAIRFAX NZ` Watercare staff Matt Hubrick, left, and Amy Holliday inspect one of the solar-panel powered buoys.

"We are not trying to get rid of the algae, just get it to levels that don't cause us problems," said Amy Holliday, water quality and environmental analyst for Watercare.

"It gives that extra assurance that when people turn on the tap the water that they are drinking is safe. ​This technology is just adding another tool to our toolbox."

Watercare believed it was the first time the technology had been used in New Zealand, she said.

SIMON SMITH/FAIRFAX NZ Lower Nihotupu Dam is one of three dams in the Waitakere Ranges Regional Park.

It was costing $384,100 in capital expenditure and, if adopted, another $20,000 each year in running costs.

The one-year trial started in December, and if it worked would save money at the water treatment plant, where less chemicals would be needed.

An American company recouped the cost of deploying the technology in 1.8 years, Holliday said.

SIMON SMITH/FAIRFAX NZ Sensors monitor the water quality.

So far it appeared to be a great success in New Zealand, with the blue-green algae count over summer 90 per cent less than the year before.

The weather in Auckland had been cooler than most years though and so it was difficult to be sure just yet.

Watercare dam technician Matt Hubrick said the cyanobacteria, also known as blue-green algae, thrived in warm water and formed slimy clumps that could become toxic to mammals.

SIMON SMITH/FAIRFAX NZ Sound waves are emitted from these transducers across the lake. The black disk is kept clean by a wiper.

"The devices we're using not only monitor the water quality, the software program interprets the changing water quality in real time, so that changes can be made almost instantly. This means the onset of algal blooms can be predicted, then crippled and slowly killed."

The ultrasonic vibrations caused its cell walls to resonate and break – similar to a glass breaking from a high-pitched sound. They did not harm humans, animals, fish or aquatic plants.

SIMON SMITH/FAIRFAX NZ Manual water samples are still being taken and analysed in a lab to compare results.