Dahokolomoki Z.
Yelp
It was a very interesting experience, having never done something like this before!
You can read the other reviews for full details, but effectively you get an informative tour about the Mt John Observatory (next door to Tekapo town), an introduction to the main stars and planets in the night sky, and then also a chance to use three of their telescopes to peer at various planets / stars. Overall, the tour is about 1hr 15mins in total I reckon (as total time end to end is 2 hours).
Do wear warm, as even though we went on a summer night, and was told it was 10 degrees up there, it did get colder as the wind picked up! And you do a lot of standing around in the open. They do provide HUGE coats to borrow for free, definitely do take one. But bring a beanie and gloves too, if you have them.
They are also rather ethical; if there is cloud cover, they will tell you before the tour starts, and you can opt for a refund. Note - this option is only for the Summit Experience, not for the cheaper Crater Experience.
On our night, we were told before the tour that the skies were 80% clear, but there was risk of some movement of clouds from nearby. We were offered the choice of a refund, but we decided to go ahead.
During the tour itself, the visibility got progressively worse. We were maybe 1hr into the tour, when they had to cut it short - the last 5-10 mins I believe. They offered a 50% refund to everyone, proactively. Which was a very fair result, IMO.
Price-wise, its not cheap at NZD200+ per person. I wouldn't label it great value for money. But it is a very unique experience.
Timing, I would say that the 10pm tour was a bit too early for peak-summer (1st Jan). The sun had barely set yet. I would say go for the tour at least 2 hours after sunset time, so you can get the full "dark sky" experience with all the stars visible.
Do book in advance if you can, as the tour groups snatch up loads of tickets 6 months out, from what I've been told.