This is your view from the top of the Diamond Lake Hike, which starts about 12 kilometers outside of the town of Wanaka. The views are so beautiful from the summit that it's nearly impossible to ever head back down the mountain.
The track starts from the car park and follows an old road to Diamond Lake. The track then climbs to a viewing platform above the lake that is a great spot for a water/snack break and some photos. From here you'll have two options:
1) A lower-level circuit that takes in the Lake Wanaka viewpoint, with great views of both the lake and its islands.
2) An upper-level circuit that winds its way to the top of 775-meter Rocky Mountain. At the top you'll be rewarded with views that include the Southern Alps and Mount Aspiring.
If you do the longer, second option, your round-trip mileage is just over eight kilometers and a good estimate for time would be three to four hours (with breaks).
Note that many walkers do both routes from the Diamond Lake viewing platform.