In reply to razzorbuzz:
I live very close to Tongariro - you can borrow an ice axe from me if you need to - and make sure you return it!
May is the beginning of winter - you will likely be happy to have both an ice axe and crampons. It is a rather long, un-sheltered walk with a reputation for shitty weather. I've done it in summer (running- won't do that again) and have skied Ngarahoe in winter(the Mt.Fuji-looking peak that you walk by) several times (which requires walking about half of the tongariro crossing before walking up Ngarahoe). Conditions can vary from absolutely miserable to rather mundane.
Right now, the 7km road from the highway to the trailhead is closed to private vehicles - you have to get on a expensive bus to be shuttled. Who knows if it'll open back up before you arrive.
Minimum kit - good walking boots with crampons to fit, warm and waterproof clothing.
send me an email at graham dot b dot johnson at hotmail. if you want more info/an ice axe/a place to crash for a few nights. I live in taumaranui - about an hour from the start of the crossing.