http://www.tongarirocrossing.org.nz/ - good site on general info with a forum to post your questions
http://www.doc.govt.nz/parks-and-recreation/tracks-and-walks/tongariro-taupo/ruapehu-area/tongariro-alpine-crossing/ - Department of Conservation information on the Crossing, including what weather conditions to expect, what gear to take and how to be prepared. It is not called and "alpine" crossing for nothing. Please respect the mountain and take heed of their advice!
http://www.thetongarirocrossing.co.nz/ - this is the shuttle we used. there are loads of transport options available, these are just one option. Their website has some fairly useful info on it as well.
Accommodation: Of course I'm not going to even try listing all the accommodation options, but the closest places to stay are Whakapapa Village, National Park and Ohakune. Note that these are very small localities and can book out in peak season (especially during the ski season, which runs from around beginning of July to the end of October). There are more options available at Turangi and Taupo, which are still within about an hour or so of the starting point. Handy tip: if you want to soak your tired aching bones afterwards, aim to stay in Taupo ;)
Enjoy!
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On the weekend, Nadene and I went and did the Tongariro Crossing in the middle of the North Is, reputably NZ's best day walk. To say it was challenging would be an understatement for us, but we are so pleased to have done it, even if we couldn't walk the next day...
Note how steep Mt Ngauruhoe is behind me!

Hmm, now do we get down from here....!

AKA Mt Doom. We made it as far as the red rocks near the top before we ran out of time to do reach the summit... :(

Nadene and I, top of Red Crater looking across to Mt Ngauruhoe
Emerald Lakes.
Nadene by and Emerald Lake
Looking across Central Crater to the Red Crater.
Reaching the last of the climbs.
Yay we made the last bus home!