One of our most internationally famous lakes, known for its pure, deep water, is under threat after an outbreak of algae.
Scientists have been monitoring a mucus-like bloom on Lake Wakatipu in Central Otago.
Queenstown fishing guide Stu Dever is regularly out on the lake catching fish, but recently he's been hauling up something called "lake snow".
It's an algal growth that looks like floating snowflakes and whilst the description sounds serene, Mr Dever says the green sludge is anything but.
In just three hours of fishing his line is clogged with the gunk.
Lake snow was first found on neighbouring Lake Wanaka in the early 2000s.
Mr Dever says it first showed up in Wakatipu in March this year and has got progressively worse. He's worried it could ruin the lake.
"I can cope with it as it is, but if it gets worse than this it'll be out of control really," he says.
The Queenstown Lakes District Council (QLDC) says whilst the algae isn't toxic and poses no public health risk, it is taking the matter seriously with reports it's affecting some of the town's water supply.
"The algae is blocking up water filters which causes a drop in pressure to residential houses or commercial sites, and those appliances wouldn't work as they should be," says QLDC chief engineer Ulrich Glasner.
The Otago Regional Council says it's about to begin its five yearly study of the lake, which it hopes will give some clues as to how best manage this algal invader.