Authorities from Greater Wellington and Regional Public Health have issued an algae warning to the residents of the Upper Hutt suburbs of Birchville, Poet's Park and those living near the Silverstream railbridge. Certain parts of the Hutt River are showing increased growth of a potentially toxic blue-green algae.

Blue-green algae are microscopic organisms that inhabit many of New Zealand's water bodies, including relatively `clean' waters. Low and stable river flows and consistently warm temperatures tend to stimulate algal growth. The algae produce toxins that can be fatal to dogs. In humans, contact with the blue-green algae can cause vomiting, diarrhoea and skin irritations.

Ted Taylor of Greater Wellington Environmental Monitoring and Investigations says that the warning is of medium risk, and has been posted to the relevant areas in order to alert swimmers, fishers and other river users. A medium risk sign indicates that people can still swim or walk their dogs but must check first for any algal mats. Blue-green algal mats are dark brown or black in colour, and tend to grow in thick layers on rocks in the riverbed.

The organisation uses a two-tiered warning sign system, which is based on riverbed coverage and algal mat exposure. This follows the New Zealand guidelines for cyanobacteria in recreational fresh waters.