People are being warned away from the water at a popular Rotorua lake because of a potentially toxic algal bloom.

Health New Zealand - Te Whatu Ora has issued a health warning for Ōkawa Bay in Lake Rotoiti.

People are advised to avoid any recreational activity in the area that involves “significant” contact with the water.

“During the warmer months lakes and rivers can be affected by blooms of blue-green algae which can lead to a release of toxins into the water,” Health NZ said.

Dr Lynne Lane, Medical Officer of Health for the National Public Health Service, said algal blooms can quickly move to other parts of the lake and multiply to dangerous levels.

“Algal blooms can sometimes produce and release toxins that are harmful to people, as well as animals, and can cause significant health issues for anyone who has contact with the water.”

Lane said lake users needed to be vigilant before having any contact with the water.

“The toxins can affect the nervous system causing numbness and difficulty with breathing, and spark asthma attacks.”

Contact with the water can also cause skin rashes, stomach upsets and visual problems.

Lane said dogs were particularly at risk of serious illness if they consumed or had contact with toxic algae.

She recommended canines be kept away from the water if it appeared there was a bloom.

“It is best to avoid contact with the water if it looks discoloured, has an unusual smell, has green or brown particles suspended in it, or if you can see scum or leathery mats of algae on the surface or on the bottom of the waterway.”

Lakes are typically dominated by free-floating algal blooms. Whereas rivers are more prone to algal blooms which grow in mats attached to rocks at the bottom. They can also accumulate at the shoreline.

“Algal mats and scum may build up along the edges of lakes or rivers so it’s especially important that parents ensure that children avoid contact with these as they may be toxic,” Lane said.

“You should seek medical advice from your GP if you become unwell after having contact with the recreational water.”

The local Medical Officer of Health issues a health warning if the results from the regional council’s water monitoring show a site has high levels of blue-green algae.

The lake is no stranger to outbreaks of algal bloom. A public health warning was issued in January this year and lifted in March.

Last summer new signs warning lake users of toxic algal blooms were put up around 40 lakeside locations in the Rotorua district as rising outdoor temperatures increase the likelihood of cyanobacteria breakouts.

Algal blooms: Look before you leap

It is always best to avoid contact with the lake water if it:

• looks discoloured

• has an unusual smell

• has green or brown particles suspended in it

• has visible surface scum.

If you think you’ve been in contact with an algal bloom:

• have a shower and change your clothes as soon as you can, even if you don’t have any symptoms

• seek medical advice from your GP if you become unwell after having contact with recreational water.