Swimmers are being urged to check for potentially toxic algal blooms before they dive into lakes, rivers and streams this summer.

Water users should also be aware of the potential for higher levels of bacteria in waterways following periods of heavy rain, and avoid swimming for up to 48 hours.

Waikato Regional Council regularly monitors blue-green algae (cyanobacteria) levels and takes samples to check for bacteria along the Waikato River and in some of the region's shallow lakes.

Summer monitoring of the hydrolakes – Karāpiro, Maraetai and Ohakuri – started on November 2 and will continue until the end of April. Monitoring began on the rest of the lakes and river sites on Monday, December 5.

The Land, Air, Water Aotearoa (LAWA) – Can I swim here? website, www.lawa.org.nz/swim, brings together algae and bacteria results from around the country. Check the LAWA web page before deciding where to go swimming, as well as the swim smart checklist.

"Algae occur naturally in our rivers, lakes and streams, and flourish during hot and fine conditions. Most algae are harmless, but high levels of some species may be toxic and blooms may result in health warnings being issued," says Waikato Regional Council's water science team leader Thoams Wilding.

"If predictions are correct and we're going to have another hot summer, there will likely be more incidents of blooms appearing. November 2022 sea surface temperatures were the highest on record, according to NIWA scientists, and all that heat translates to warmer growing conditions in our lakes and rivers."

Thomas says blue-green algal blooms are often found in waterbodies throughout New Zealand during summer months.

"Here in our region, blooms have been recorded in the Waikato River hydro lakes and Lake Taupō. Some of the shallow lakes, such as Waikare, experience more frequent blooms.

"It is important that you use your own judgement before getting into the water. Algae can accumulate rapidly around the shore, despite it being clear the last time we sampled. If the water is bright green like pea soup, stay out.

"Extra care should be taken with toddlers and dogs as they like to put things into their mouths and even a small amount the size of a 50 cent piece can be enough to cause serious harm if eaten."

Te Whatu Ora – Health New Zealand's National Public Health Service works closely with regional and district councils in the Waikato region to provide health advice, and Medical Officers of Health issue health warnings when monitored lakes are unsafe for recreational use.

Te Whatu Ora Waikato Medical Officer of Health Dr Richard Wall recommends that where cyanobacteria health warnings are in place, lakes should not be used for any activity that involves skin contact with the affected water.

"Swallowing water from lakes affected by blooms should also be avoided.

"Scums are a particular risk because they contain a high level of toxins. If contact with scum does occur, skin should be rinsed clean and clothing changed as soon as possible. This warning is particularly important for children.

"If people still choose to use the lakes when warnings are in place, or any other lake where there are visible changes to water colour, they should shower and change their clothing as soon as possible afterwards, even if no symptoms are noticeable."

Symptoms can include rash, skin and eye irritation; allergy symptoms such as hayfever and asthma; and possibly stomach upsets including diarrhoea and vomiting.

Warnings will be issued for the lakes that less commonly experience algal blooms.

Thomas urges swimmers to check the lawa.org.nz/swim website before swimming, but adds that it will only give an indication of what water quality was like at the time of sampling.

"That's why it's so important to check the water before you dive in."

If you become sick after swimming, contact your health professional, and let them know where you went swimming.

Waikato Regional Council's environmental monitoring will be a little different this year. As well as the algae recreational monitoring, measurements on the colour of water will be taken using a hand held device.

"It provides an instant reading on the types of algae in the water, based on colour, and is a method that we're trialling alongside the more in depth lab analysis in the hope we can give an even faster result."