Subway currently have an offer of 12 free cookies with any catering platter ordered (Posted by nachos)
Apparently the 12 cookie catering platter is included as a 'catering platter', so you buy 12 cookies and you're eligible for the 12 free. This is shown at the checkout page after you add 1 x cookie platter added to your order is: "2: FREE COOKIE OFFER (1 X 12 COOKIE PLATTER/S) ($0.00)
You can place your order online or instore - although i'd expect it's easier to claim the offer by ordering online than trying to possibly explain the offer to store staff.
Valid at participating stores only - I tried online ordering at a number of stores and it seemed to replicate.
Please note - platter price varies between stores, Dixon St Wellington City has the platter for $7.80, from other user reports other stores have prices ranging up to $11.60