The toxic algae bloom affecting Lake Taupo is expected to stay around for as long as the hot, calm weather does.
Even if it is broken up by waves, wind and rain, it may recur if conditions turn hot and still.
Taupo mayor David Trewavas said at a press briefing today it was the first time he had ever hoped for rough conditions on the lake.
"A rough lake and a bit of wind and a bit of rain might hopefully disperse it."
Waikato Regional Council water scientist Dr Eloise Ryan said significant rain and cooler temperatures were needed to slow the algae growth.
"In my experience we won't see the bloom disappear ... If these conditions persist, we may see the recurrence of this bloom over summer."
The regional council is collecting more water samples today and will send them to Niwa and the Cawthron Institute to analyse.
Contact with the toxic algae bloom can cause health effects such as skin rashes, irritation, breathing difficulties, stomach upsets, tingling of the hands and feet, numbness, muscle weakness and more serious difficulty breathing.
Dr Ryan said the algae may not yet be producing toxins but could do so at any time so the council would continue regular monitoring.
The water would be tested weekly, she said.
Taupo District Council will test drinking water intakes daily.
Dr Ryan said while the algae Phormidium is naturally occurring, to get blooms established it needed other factors such as nitrogen and phosphorus.
"We've got ideal conditions here in Taupo at the moment with hot dry weather with sufficient nutrients to support such a bloom."
Medical officer of health Dr Neil de Wet said just because not all lake areas were listed as no-go zones, it did not necessarily mean they were safe, and people should be vigilant.
"Even in those areas of the lake where we don't yet have information or haven't yet issued a warning, be aware and look before using the lake because blooms can occur rapidly. It's important you check and if there's any visible algal material or any strange odour from the lake it's best not to use the water if there's any doubt."
The algae forms brown-green mats or blobs in the water, which are easily spotted.
Dr de Wet said it was unlikely there was any risk from eating trout as long as people did not eat the gut, however they should not eat koura (freshwater crayfish).
There have been no reports of health effects so far and clinical staff and doctors throughout the area would notify him of any.
He was confident that the Taupo District Council's drinking water supplies were safe.
"They are closely monitored and managed. There are possibly some small individual private household supplies that the council doesn't know about and our advice to anyone who has one of those would be to shut it down and find another source of water during this period."
Tuwharetoa Maori Trust Board chief executive Topia Rameka said given that water could take up to 80 years to reach the lake, the effects of fertilisers and farming from the 1960s were only being felt today.
He said the effects of removing nitrogen from Lake Taupo in recent years would not be fully apparent until as far out as 2080.
"The water quality that we are experiencing now in Lake Taupo is a reflection of land use 40 to 60 years ago," Rameka said.
"When we undertook the Lake Taupo Protection Project and removed 170 tonnes of manageable nitrogen from the lake we always knew there was a significant load of nitrogen going to come through and it would eventually be caught up by the nitrogen decrease.
"This was known back in 2000 when we commenced the Lake Taupo Protection Project. Things like what we are experiencing now were well known at that stage and it's something the community needs to be a little bit more informed about."