Change the Google search string

I want to change the URL that is used to bring up Google when I search 
from the location bar, to use Google.co.nz instead of Google.com

Using Multizilla's Qprefs menu I opened the preference editor 
(about:config) and found two settings referring to Google:

browser.search.defaulturl
keyword.url

I altered both of these but this has not changed the search function, 
which still loads Google.com