Real Journeys senior guide Justin Remnant said the Te Anau Glowworm Caves were looking "really amazing" after the glow-worms had multiplied in large numbers.
"The cave ceilings look as if they are covered in supernovas as the worms hatch and disperse from the egg clusters.
"Right now, the glow-worms are so bright you can see the faces of people sitting in the boats looking up at them," Mr Remnant said.
"Everyone's just loving it."
The company was particularly thrilled, because heavy rain in April flooded the caves and wiped out large numbers of glow-worms.
While the timing could not be better for visitors to Te Anau over the holiday period, those wanting to see the show put on by the babies had just two to three weeks to do so, Mr Remnant said.
"As the tiny glow-worms get bigger, the strongest of the bunch will eat its neighbours to make a two to three centimetre territory of space around them.
"They're still going to be impressive even after that because there are just so many."
