I have had a look through various websites looking for sources of ammonia for NZ, no go so far am afraid.
Nearest online source of ammonia is in Australia but might as well be on the moon for you!
Also had a quick look at the Prohibited Import on New Zealand Customs Services, it appears that approval to import to New Zeanland is required, and think for fish keeping hobbyists its not high on their priority list and would be extremely surprised if one was given approval since Ammonia is a Hazardous Substance due to Corrosiveness.
http/www.customs.govt.nz/features/prohibited/imports/Pages/default.aspx
The only things i can think of is alternative sources such as the link TTA provided, or asking your fish keeping friends or other friends / family if they have any household ammonia, have heard cloudy ammonia has been used but unsure about that.
Perhaps doing a ammonia rich water, something like putting in a quantity of raw prawns into a container of water and allowing this to decompose over x amount of time and sieving out the contents of raw prawns and you would be left with ammonia rich water when you could the test the ammonia rich water to see how much ammonia that water contains and working out doses from there, possible imo.
Not the best but better than nothing or do as other have done, add prawns or fish food to your tank but a highly inaccurate and messy way of doing things.