Is right here,
at Te Waikoropupu
Springs, or as locals
affectionately know it,
The springs are located
in the Golden Bay
region of our Tasman
And yes it does have officially the clearest water in the whole world.
They pump out a combined 14,000 litres of water a second, or 1,200,000m³ each 24hrs.
As Wikipedia say
The horizontal visibility of the water in the springs has been measured at an average of 63 metres, a world record for fresh water
In fact so clear that, with optical instruments, scientists have said it approaches the theoretical maximum for optically pure water. If you are interested in the full terminology for “average black-body visibility” then I believe the above links will satisfy your craving.
Famous photo over here of diver under the water.