The Canterbury District Health Board is warning river-goers to avoid a potentially toxic blue-green algae.
The warning applies to the Waipara River, in an area about an hour north of Christchurch.
The public should avoid the area, as the toxins in the algae can cause skin rashes, nausea and stomach cramps.
The DHB says dogs are particularly vulnerable and should be taken to the vet immediately if they ingest any of the algae.
The Canterbury Regional Council is monitoring the sites.