UPDATE 4 Feb 2020: Regional toxic algae alert issued by Greater Wellington

Toxic algae is quickly intensifying throughout the region, with all monitored rivers reaching health alert levels and Wairarapa’s Waipoua River remaining hazardous for swimmers.

Greater Wellington’s Otaki, Waikanae, Hutt, Ruamahanga and Waingawa river monitoring sites have exceeded the 20 per cent alert threshold, and the Waipoua River is above the 50 per cent  “no swimming” line.

Detached mats, which are particularly hazardous for dogs, have been seen in all rivers and it is highly likely the same conditions apply to non-monitored rivers in other parts of the region.

“Our message is clear. People should stay out of the water and be careful at riversides to protect toddlers and dogs from contact with toxic algae,” says Greater Wellington Senior Environmental Scientist Dr Mark Heath.

“Levels are expected to increase, with hot dry conditions forecast for much of the region throughout next week.

“This comes on top of a prolonged dry spell. With the exception of the Otaki River, it’s been 35 days since the last decent flush for most rivers and streams in the region, which has produced ideal conditions for the growth of toxic algae.”

Warnings are being placed on all the monitored rivers and sites, and a range of media is being used to inform the public of risk levels around rivers.

Specific site warnings are:

  • Otaki River, warning extends from State Highway 1 bridge to river mouth
  • Waikanae River, warning extends from Main Rd (old SH1) to river mouth
  • Hutt River, warnings for Melling and Siverstream sites
  • Waingawa River, warning for south road site
  • Ruamahanga River, warning for Morrisons bush site
  • Waipoua River, warning extends from Paierau Road to confluence with the Ruamahanga

Toxic algae grows on submerged river stones, presenting with a shiny brown/dark green to violet coating. It can also become unstable and detach, floating to the surface forming small brown/black mats at the water’s edge.