Health officials in Northland are assessing the risk to people from a large algal bloom.
Red seaweed has been washing up on the coast from Ruakaka to Langs Beach.
Since last weekend's earthquakes it has built-up in Waipu Cove and Bream Bay, killing eels and fish in the lagoon.
It's a good job you're watching this and not smelling it because the stink is terrible.
"It's just putrid. It gets in your throat, gets in your nostrils," says Ellen Snelling, from Learn to Surf Waipu Cove.
"I don't even know how to describe the smell but think like an old drop toilet and times it by ten."
A red algal bloom has washed into the lagoon from Bream Bay and became trapped.
"The problem is it gets washed up on the rocks, then it putrifies in the sun and then it gets washed back in," Snelling says.
As it rots the algae sucks the oxygen out of the water, killing eels and fish and generating the sulphurous 'rotten eggs' gas that is wafting over the lagoon.
"We had builders up here a couple of days ago and they had to come off the roof. They said they felt like they were going to vomit," Waipu resident Voss says.
The algae itself isn't toxic or harmful but the District Health Board (DHB) acknowledges some people have suffered eye irritations and headaches.
The DHB says it's more of a nuisance than a danger. However, it has been advising people to stay clear and upwind of the source, and if you're feeling unwell to get away and seek medical advice. Locals should close their windows.
Now it's also telling people to avoid swimming and kayaking, keep children and dogs away and don't eat shellfish from the estuary.
Coastal engineer Andre Labonte suggests the scallops, tuatua and crabs that would have filtered out the algae have all disappeared over the years just as effluent builds up with more people moving to the area.
"Too many fish and not enough oxygen and then it all starts to collapse," he says.
"And the fish are gasping as you saw for air and for oxygen and eventually pretty much everything ends up dead if it can't get out."
Residents want the entrance cleared immediately, but the council says the problem's likely to persist until the algae has broken down or is flushed out by an autumn storm and is likely to get worse before it gets better.