Garlic mustard cotyledons are long and skinny, not the shape shown in Di's pic. Here's a pic of a thick stand of Garlic mustard seedlings that I found online. They are just getting their first true leaves and you can still see the shape of the cotyledons on a lot of them:
http://www.kingcounty.gov/environmen...seedlings.ashx
Are there any wild brassicas in New Zealand?