An Auckland couple who came across what looked like "black sewage" at Titirangi Beach say it smelt so terrible it made their eyes water.
Saoirse Chapman and her partner Nicholas Calland were at the beach at 11am on Monday when they noticed the stench.
"First we noticed the smell - basically like an overflowing blocked toilet," Chapman told Newshub.
"As for the colour, I've never seen black water before... It felt apocalyptic."
She says there was a stark contrast between the colour of the water and the kererū and tui singing in the trees.
"The wildlife didn't contribute to this pollution and are the ones swimming, feeding and breeding amongst our sewage. If we don't clean up our act immediately there won't be any wildlife left."
A video Chapman sent to Newshub shows black water flowing down a stream that connects to the beach.
Titirangi Beach is listed as a 'do not swim' beach on Auckland Council's Safeswim website due to its poor water quality.
The couple has been to the beach several times before and Chapman says they're both aware it's known as an unsafe place to swim, but this is the first time they've seen and smelt the sewage.
Auckland Council Safeswim programme manager Nick Vigar says the "pungent smell" in the stream isn't sewage but is instead stagnant water that hasn't been flushed out by rain.
Organic material such as grass and leaves rot in shallow water. This process uses up oxygen in the stream and produces the strong-smelling and dark sulphur dioxide.
"Although it looks and smells unpleasant, it is a natural process and will be restored when there is more rain," Vigar says.
If harmful substances are suspected to be polluting the stream, Vigar asks the public to call the council's pollution hotline on 09 377 3107.