Blue-green algae found in Hutt River

Green algae is seen on the main beach at Mount Maunganui. File Photo / APN

Authorities are warning people to take care around parts of the Hutt River following increased growth of potentially toxic blue-green algae.

Greater Wellington and Regional Public Health have urged care, after monitoring identified moderate growth of the algae in the river at the Upper Hutt suburbs of Birchville, Poet's Park, and near the Silverstream railbridge.

Greater Wellington environmental monitoring and investigations manager Ted Taylor said medium risk warning signs were being posted in those areas to alert river users.

"We use a two-tiered warning sign system, based on river bed coverage and algal mat exposure, to advise river users of the risk from toxic algae which follows the New Zealand guidelines for cyanobacteria in recreational fresh waters."

A medium risk sign meant users could still swim or walk their dogs, but should check first for any algal mats.

A high risk sign meant people should avoid contact, recreation, and dog walking in that part of the river.

Blue-green algae are microscopic organisms naturally present in many New Zealand waters, including relatively 'clean' waters.

Algal growth increases when river flows are low and stable and temperatures consistently warm.

Blue-green algal mats are a dark brown or black colour and grow as a thick layer on rocks in the riverbed.

The toxins produced by the algae can be fatal to dogs, which are particularly susceptible because they like to scavenge near water.

Contact with blue-green algae toxins may also cause vomiting, diarrhoea and skin irritations in humans.

- NZPA