Growth of blue-green algal mats presents high risk
Greater Wellington and Regional Public Health are asking the public to avoid contact with the Hutt River at Birchville
(Upper Hutt) due to extensive growth of potentially toxic blue-green algal mats.
Large amounts of blue-green algal mats have come loose from the river bed and washed up on the river’s edge where they
are easily accessible to dogs, stock and recreational users.
High risk warning signs have been posted at affected sites in the Birchville area. A high risk sign means people should
avoid contact recreation and dog walking in that part of the river.
Moderate growth of blue-green algae is present along the remaining length of the Hutt River downstream of Birchville and
medium risk warning signs are in place. A medium risk sign means users can still swim or walk their dogs but should
check first for any algal mats.
The warning system is based on river bed coverage and algal mat exposure, and follows the New Zealand Guidelines for
Cyanobacteria in Recreational Fresh Waters.
Blue-green algae are microscopic organisms that are naturally present in many New Zealand waters, including relatively
‘clean’ waters. The algal mats are actually a dark brown or black colour and grow as a thick layer on rocks in the
riverbed.
The growth of the algal mats follows a very dry November with low rainfall in the Hutt River catchment in November and
declining river flows.
More information about toxic algae, including pictures and current warnings, can be found at www.gw.govt.nz/toxic-algae. If you see any exposed or easily accessible algal mats, contact Greater Wellington on P 04 384 5708 or an
environmental health officer at your local council.
ENDS