Waitomo Glowworm Caves (Martin Rietze) Photo:
This story has been updated because NIWA now says there have been no confirmed sightings of koi carp in Waitomo Caves.
NIWA scientists had to resort to some old-fashioned tools to help prevent unwanted koi carp from reaching Waitomo Caves.
There has been concern koi carp might get into the caves and threaten indigenous species and freshwater habitats.
NIWA scientists were called in to help with the preventative work so the fish could not get into the caves which are a major tourist attraction because of their glow-worms.
NIWA principal scientist for natural hazards and hydrodynamics Graeme Smart told Morning Report koi carp were released into Waikato rivers many years ago and their population had gradually increased.
"I don't think there's any survey been done on the numbers but they don't want any there. They're voracious feeders, they want them gone."
The Waikato Regional Council planned to build a weir (or fish barrier) downstream of the caves to stop the upward migration of the koi carp.
NIWA staff were trying to to survey how much of a fall in water level there was between the upstream and downstream side of the cave and if the downstream weir would cause more flooding.
"We had big problems. Conventional survey equipment won't work in a cave - it needs to see the satellite for its GPS to work," Dr Smart said.
So they had to bring out the old-fashioned staff and level "where you look down a telescope like a big measuring stick" and go into the cave in a boat in the dark and survey the water levels right through.
"The cave disappears into a tunnel halfway though the hill and so to link up the upstream and downstream side of the cave we had survey points all the way up across the top of the hill and then down to the resurgence where the water comes out again.
"But the shortest route was through the cafe of the Waitomo Caves Visitors' Centre so we had to move through amongst all the customers sipping their lattes and say 'excuse me, can we just place our staff beside your table' to link these upstream and downstream water levels."