To get to the waterfall you drive to the barrier arm at the beginning of the road that leads you down to the Lady Knox Geyser at Wai-O-Tapu (Note, the barrier arm is closed after the geyser has finished in the morning approx 11am, so you cannot drive your car down). Park somewhere near the barrier arm (but not in front of it!) and walk for 5 minutes down the road until you are almost at the entrance gate to the geyser. You will see a track just after you pass the stream that goes underneath the road. The waterfall is just 1 minute walk from the road. If there are no cars when you parked up at the barrier arm will mean you will be the only one there so swimsuits are optional.
My advice is take a pair of Crocs or flip-flops because the mud/sand on the bottom can be very hot in places – almost too hot to stand, particularly directly under the waterfall (so I found out!). The water is only a few feet deep and has a grey thermal mud bottom, so if you are expecting crystal clear water to bathe forget it here – it’s rustic and natural as it gets. Also note the Lady Knox geyser is under camera surveillance, so don’t even think about trying to get a free look at that after your swim or security will be after you like a shot.