There is nowhere on the Tongariro Crossing trek to refill water bottles, so take at least two liters if you’re hiking the main trail, and even more if you’re planning to summit Mount Ngauruhoe or Mount Tongariro. You will need it, even on cool days.
Bring high-energy snacks like trail mix and cereal bars, a few pieces of fruit, and a sandwich or two. I always find I bring way more food than I need, but if you have too much you can always share with the new friends you’ll meet on the trek.
Since New Zealand is situated under a big rip in the ozone layer, even easily tanned folks need constant reapplications of sunscreen. There is almost no shade on the Tongariro Crossing, so you can expect to spend 6-8 hours in the beating sun if it’s a clear day.
If you have any issues at all with your ankles, knees, back – whatever – be sure to take along plenty of your medication of choice. And bring bandages for unexpected blisters which almost certainly will find their way onto your toes.