Heavy rain earlier this month has resulted in bacterial contamination at a number of popular swimming and seafood gathering locations in Northland.
The Northland Regional Council monitors most of the region's popular swimming spots from November until the end of March.
Northland DHB Medical Officer of Health Dr Jose Ortega Benito said recent testing had indicated the presence of bacteria in a number of locations, prompting warnings against swimming or gathering shellfish.
"It is important to note that the test results are only a snapshot of conditions at the time of testing, and if there has been heavy rain since, it's likely the water quality will be worse," he said. It was also unwise to collect shellfish after heavy rain, which could flush sewage overflow or farm run-off downstream.
Shellfish should be safe to collect after the water has run clear for a few days, he said.
The DHB's advice was not to swim for 48 hours after heavy rain (more than 10mm in 24 hours), to read signs at rivers and beaches carefully, and not to enter the water if signs advised against swimming.
Salt water was generally safer than fresh water, due to the pathogen-killing effect of salt.
For bacteria and viruses, the sea was usually safer than a lake or river.
Moving water was also generally safer than still water, so a river would usually be safer than a lake, and sea water on an open coast safer than a harbour.
"Even if there are no warning signs there may still be some risk," Dr Benito said.
"Use common sense, as a range of environmental factors can affect the quality of recreational water.
"Consider what might flow into the area you intend to swim in, such as stormwater from outfall pipes, stormwater run-off, stock waste, failing septic tanks and boats emptying their toilets.
"Look at the water for signs of contamination, such stagnant, muddy or cloudy water.
"If the water is cloudy, there is visible scum, an odd smell or colour, or you cannot see your feet in knee-deep water, it may not be safe to swim."
* Test results can be seen at www.nrc.govt.nz/swimming