US sites will show up fine on a search of google.co.nz. I don't understand what you are expecting to happen? This is exactly how Google should and does handle sites.
About 8 months ago it was exactly the opposite with my rankings. I was nowhere to be seen in google.co.nz but was #1 at google.com for my main keyword. This is what I would have expected from Google for a site targeting world wide audience. For some reason I am now ranking #1 on google.co.nz
Could be that you have gained a lot of .co.nz links to the site which is helping to geo locate you as being relevant to a search there. This is how Google works. It is not JUST what they think is relevant, if enough country specific sites link to you then fair to assume that your content is relevant to that country.
This would make sense but definitely not the case with my site. Last thing I can think of is changing the name server which pointed to a New Zealand registrar. I am changing over to US registrar tomorrow and see what difference it makes. Thanks for all your comments and advice.
IP address in my opinion definitely doesn’t make any difference. Having my own server I can buy a block of IP addresses from anywhere in the world. On another US server I have the assigned IP addresses show up as a South Africa location. Questioning the hosting company I was told the IP addresses where purchased together with a company they bought. The sites on that server ranked best on google.com attracting mainly US visitors.
When you try to rank in uk and have more links coming from .co.uk sites to your site You may rank well in yahoo uk and google uk., TLD links do matter to rank well in that particular country, As to how google determines the country of a website., a)TLD links, b)Country where a site is hosted, c)It can also derive this info from the Users having the google toolbar (I assume so), For example if i start a india related site , It will be visited mostly by indians, Google may know about this through its toolbar.,
I know I sound like a broken record now but my site has the following: Extension: .com Server location: US Incoming links: estimated 90% US Visitors: 80% US