Dept of Building give a yearly estimate on m2 costs for building.
http://www.building.dbh.govt.nz/e/pu...ng_costs.shtml
Also alot of kitset houses work out at $500m2 to lock up stage.
www.carters.co.nz are a building supples place and they do ready made timber frames and roof trusses.
www.gerardroofs.co.nz give an excellent guide on cost of roof.
I reckon that it is ok to build your own if you need plumbing to sewer mains you have to get Nz certified plumber...all the new building law rules can be found on
www.consumerbuild.org.nz
you can also buy seconhand windows and doors on
www.trademe.co.nz