There is a reciprocal health agreement between the UK and NZ, this will provide some medical cover for you in NZ, here is a link to info on it (Copy and paste it into your browser).
http://www.nzembassy.com/info.cfm?c=14&l=56&CFID=402&CFTOKEN=86934083&s=go&p=61688
You could take out a twelve month travel policy but it will be hundreds of pounds which I assume is to much.
If you want things like personal accident cover, this can often be taken out on the slopes, if you want cover for your contents and sports equipment you could probably take out a contents policy over there which I would assume would be cheap in NZ.
If you want cancellation cover for your out bound flight you cannot just take out a normal short period travel policy as they are subject to you leaving the uk and returning to the uk within the period of the insurance. What you will need to take out is a "One Way Travel Policy" Here is a google search for "One Way + Travel Insurance" (Copy and paste it into your browser)
http://www.google.co.uk/search?hl=en&client=firefox-a&channel=s&rls=org.mozilla%3Aen-GB%3Aofficial&hs=EJj&q=%22one+way+%2B+travel+insurance%22&btnG=Search&meta=cr%3DcountryUK|countryGB