Toxic algae is taking hold in rivers in the Greater Wellington region in the long, dry spell, rendering some waterways off limits.
The worst affected is Masterton's Waipoua River, which has passed the 50 percent alert level, meaning swimming is banned.
Regional council monitoring of other river sites has shown levels above the 20 percent alert level.
Council environmental scientist Mark Heath said he had never seen a bloom this widespread.
With no rain in prospect the toxic algal levels were likely to deteriorate further.
"We're advising people to know what toxic algae looks like and if you see it, to avoid it."
Detached algal mats, which can be deadly for dogs, have been seen in all rivers, Heath said.
Warnings are in place for a Southland river after it was found to have increased levels of toxic algae.
People are being urged to avoid swimming or having contact with the Waikaia River until the warnings are removed.
Otago fire bans
A forecast of extreme fire danger in the dry South Island conditions has led to a ban on all fires in Central Otago and the Queenstown Lakes districts.
Fire and Emergency said banned activities included welding, grinding, chainsaw use or mowing roadsides.
Principal Rural Fire Officer Graeme Still said all those activities have the potential to start a fire during dry conditions.