Updated 1.31pm: New DNA tracking techniques are being used to find the source of a contamination at a popular Auckland holiday spot.

Martin Neale, Auckland Council's environmental science manager, says the pathogens and bacteria found in the lagoon pose a particular challenge.

"We can tell they're in the water but what's difficult is determining where they're coming from. So there is some ongoing work and we're using some relatively new technology based on DNA."

Martin Neale says new DNA tracking is being used to find out where the contamination's come from.

"Different types of animal have different types of bacteria in them, and we can use the DNA signature to tell what kind of animal it's coming from."

Martin Neale says the DNA method costs hundreds of dollars per test, compared to around $20 per routine bacterial test.

But he says it's proven successful in the past, and work is underway to make it cheaper.

Warning to holidaymakers

People are being urged not to swim or collect shellfish at Auckland's Piha Lagoon, amid fears it's been polluted by leaky septic tanks.

Dr Cathy Pikholz says drinking water which has been contaminated by wastewater can lead to gastrointestinal illnesses.

These include norovirus, hepatitis A, salmonella and campylobacter.

Dr Cathy Pikholz says people need to take the situation seriously.

"At any beach or lagoon where there are warning signs around, people need to observe those warning signs and are strongly advised not to swim, paddle or enter the water."