ABSOLUTELY AMAZING. I have to confessed, we decide to do this hike because of "mount doom" and after completing it we were amazed with EVERYTHING. we did it around January 4-7, cant recall the exact day by now :/

We took a shuttle from our RV campground to Mangatepopo car park; and the same service pick us up at Ketetahi car park. (I think we used this one https://nationalparkshuttles.rezdy.com/190355/tongariro-alpine-crossing-return-transport-from-national-park-village-to-the-mangatepopo-car-park )
At the start it was misty and light rain, by the time we hit the real steep part it was foggy; we stay around 1h in the highest point, and when going down it was sunny.
Our shuttle pick us up around 9am, and we got the last shuttle back at 6pm. Took around 1h on the top and took a LOT of picture. we (I) are also slow pace hikers.

Is suppose to be very crowded, dunno if it was because there was storm in the area the day before (we had to re-schedule our trip to the area and stay in Taupo an extra day) but it never felt crowded.

Advice: Make sure you have rain jacket, do layers (I was wearing 1 t-shirt, 1 swearer, raining jacket; 1 legging and one water proof pant on top >> end up with leggings and t-shirt; swearer on and off)
We took 2L water each, but it was not as hot/humid so we consume less water than on previous hikes.
Good boots
DONT forget your camera :D

Enjoy