I just found out I'm going to NZ in Sept. I have a conference in Tongariro National Park and I'd like to take advantage of the time there to do some walking. The round the mountain trail sounds like a good fit but I'm not sure what the weather and snow conditions will be like in early Sept. Any locals have advice on route selection, alternative treks and other areas to consider. Does NZ have any rugged coastal hikes in the 150KM length range that would be in condition that time of year?

ask the guys at http://tramper.co.nz/?view=forums
they can help you with all your questions.

there will definitely be snow at that time of the year. note that NZ is known for strong winds (70 mph is nothing unusual), so you should be prepared for severe conditions. That nice "walk" around the mountain could easily turn into a walk in nasty weather. But it seems that the weather is usually better during winter.

I don't know this area that much, the DOC homepage is another good place to check out:
http://www.doc.govt.nz/

Certainly anything up on the three mountains Ruapehu, Tongariro, Ngauruhoe, the alpine crossing (which is part of the round the mountain walk) is going to require iceaxe and crampons. Spring snow in New Zealand is very variable and is often quite wet and icy. AS noted above spring often has a lot of wind. The lower level tracks in the bush would tend to have less snow.

Other options nearby are the Kaimanawas and Kawekas to the east, Kaimais to the north, and Ruahines to the south. All these options except the Kaimais would have the tops covered in snow, but the valleys are likely to have less/little snow. These areas are much less developed than Tongariro NP, the tracks are rougher/more difficult, the huts smaller/less well equipped but cheaper, also far less people around. Transport to these ranges is more difficult too, when compared to the number of shuttles/buses available for Tongariro NP.

Long coastal walks are pretty rear in NZ. There's 90 mile beach (3 days) about 8 hours drive from Tongariro, or the Heaphy track or Abel Tasman in the South Island.