Andib:kandjc: A salesman at the Spark shop told me the Vodafone Prime 6 will not work on Spark network.
They are correct AFAIK, Looking at the specs it doesn't support 850 WCDMA "3G" so will have very limited coverage on Sparks Network outside the main centers that have 2100mhz infill coverage.
I believe it does support 850Mhz, could be wrong, if someone who knows can chime in.
EDIT: Vodafones data sheet confirms it supports 850Mhz, and its on the box.
http://www.vodafone.co.nz/MEDIA_ProductCatalog/m780073_Product_Datasheet_1_14_V895.pdf