Hot temperatures and high nutrient levels have increased the chances of smelly toxic algae blooming at a north Auckland lake.
Auckland council said while none had yet been seen at Lake Pupuke on Auckland's North Shore, that could "rapidly change" as temperatures continue to increase.
A bloom would cause the lake to turn a murky brown and release an offensive smell, affecting the lake's many recreational users and lakeside residents.
To get the jump on any potential change, Auckland Council has applied for resource consent to be able to drop flocculants into the lake at short notice to disperse a bloom, if it happens.
Flocculant is a clarifying agent, used to treat waste water.
An Auckland spokeswoman said this would not the first year the lake has fallen foul to the bloom.
Divers have been testing oxygen levels at the bottom of Lake Pupuke in order to determine the likelihood of a algal bloom occuring.
"The problem has recurred every year since 2014, impacting the use of the lake".
The spokeswoman said Lake Pupuke's impressive depth, combined with a lack of feeding or draining waterways, would create a "sink" effect that would add to an algal bloom's complex nature.
The water is monitored regularly and it is currently safe to swim in, she said on Thursday.
No algal bloom has been detected yet, but could occur very soon.