People visiting the Kerikeri Basin near the Stone Store - one of Northland's iconic tourism and heritage sites - could come face-to-face with a sign warning them the water is polluted.

The public are being warned not to swim at a raft of Northland locations after they failed water-quality testing the week before Christmas.

The Kerikeri Basin is one of them. There is such widespread failure of freshwater quality after the recent rain that district councils are not putting up warning signs at some sites.

Half the 28 tested returned high enough readings of E. coli bacteria to be declared unsafe. The 14 failed sites, mainly freshwater streams, had E. coli counts of more than 550/100ml water. Others received orange-alert warnings for levels above 260/100ml.

Also failing the safety test in the Far North were Ahipara by the motor camp, Skudders Beach, Aurere Beach stream, Coopers Beach stream, the Kapiro Bridge-Pururua Rd stream, Tirohanga Stream, Taipa, Opononi, Twin Bridges, Victoria River, Otaua Stream, Waipapa basin at Charlie's Rock and Waitangi Bridge.

In Whangarei, red cards apply to Ocean Beach stream, Otamure Bay stream, Waipu Cove Stream, Ruakaka Stream, Matapouri Bay bridges, Kowharewa Bay, Ngunguru Motor Camp and Ohawini Bay. Several others including Whangarei Falls and Raumanga Stream have orange alerts.

Unfortunate as it is that popular spots are marred by pollution, the warnings are necessary "and at least honest", says eco-tourism operator Jeroen Jongejans.

"Everybody is upset about this, not just councils and tourism operators but everybody who wants to believe in a clean, green Northland," he said.

"I think that 20 years ago, we had a worse problem but we didn't know about it then. Now we're demanding higher standards and more action.

"We're working on it but there is no quick fix. The reality is you'd find similar results anywhere there were animals and humans," said Mr Jongejans, who is on the Northland Conservation Board, Whangarei District Council and the Tourism Development Group.

Medical officer of health Jonathan Jarman was out of the district and asked for time to examine the results before commenting.

Swimming in contaminated water can cause skin, eye and ear infections and stomach and respiratory illnesses. Bacteria can be swallowed or inhaled and can also enter the body through cuts and scrapes.

People should avoid swimming in freshwater streams two to three days after heavy rain.