The only time I have seen fresh watercress has been at Moore Wilsons in Wellington - however I am sure it must appear at various farmers markets.
Check this article out - a very cool site: http://urbanforaging.manukadesign.co.nz/index.php/2009/08/10/wild-watercress/ the watercress soup recipe looks great - lovely colour.
Wild watercress is something you have to be very careful with - as you could say watercress is an amplification of the water quality the plant is situated in. For instance I had a friend who got very sick eating watercress from a roadside stream - this was in the days when petrol had a higher lead content.
To prove my point this Niwa article talks about the use of watercress to clean up water contaminated with dairy runoff. It describes watercress as "a ‘luxury’ feeder because it can absorb more nutrients than it needs to survive".
So therefore, do be careful, watercres must be taken from pristine water - or again you can buy it commercially from people such as:
Ok assuming you have found a safe supply here are some recipe links:
Prawns and Watercress salad: http://www.kingfisher.co.nz/recipes/recipe_detail.asp?rid=1
Watercress and chicken soup: http://recipefinder.msn.co.nz/article.aspx?id=849420