I hadn't read all the comments before setting off, and I regret it. You absolutely must read the comments about the route because, indeed, the Google Maps route is impassable for a non-4x4 vehicle.

Free campsite (according to DOC website), two long-drops which were clean and not too smelly. Covered picnic table by the lake. Weather changed drastically while we were there but beautiful in the sunshine! About an hour drive on a gravel road from Hanmer. The road to St Arnaud is only accessible for 4WD (rocky with lots of deep creeks) and has a 60$ fee, if the road is open (certain seasons). Limited phone signal at the campsite, but decent signal about 1km up at the entrance.

Nizza

Challenging road with lots of holes, stones and dust (4WD ideal). Costs $60/car to get to it if you go via Rainbow station (from St Arnaud).

Awesome spot in nature. Plan to spend at least 2 nights at Lake as the drive from Hanmer springs is not an easy one. Smart to have reliable spare tyre & tools in case of flat. Also no network coverage entire way. Drive slowly (30-40Km/h) and carefully to avoid potholes & large/sharp rocks and you will be fine.

Stayed here for two nights, the drive is definitely worth it! Stayed there two nights, first night had the place to ourselves, the toilets are super clean. Weather changes fast up there, can be very windy. Could have stayed there much longer. The lake and surrounding ranges are amazing!

What a lovely spot. We just had a bit bad luck with the weather, it can get crazy windy up there. Plan enough time to drive with a 2WD, the road is pretty bad in some spots. Lots of potholes and corrugation. Toilets clean and not really smelly. One had some toilet paper the other didn’t :) just bring your own. It’s really beautiful up there!

Probably best free camp in NZ. As comments below said, don't take Google map road. Gravel is quite challenge but it's for sure worth it to go there. There is also quite a lot of sand flies but where they are not. Simply amazing

Gravel road the whole way. Took 2-2.5 in camper. Check the weather, not as amazing on an overcast day. Would not recommend overnight as the winds made it near impossible to sleep.

DO NOT FOLLOW Google maps. Take Clarence Valley Road (there is a sign indicating the lake so you know you're on the right road). Long but mostly good gravel road with stunning views. Very underated place, it was one of my favorite spot in New Zealand! Super remote and calm (but cold, bring jackets!). Loved it.