Just received this email, looks like a generic code. Discount stacks on top of existing promotions as far as I can tell.
20% off and free shipping on your next purchase offer only valid with discount code PB4275. 20% off your next purchase and free shipping expires on Sunday 20 September 2015. Offer only valid for online orders at www.petbarn.com.au. Limit to one use per customer. Maximum cart amount to which this discount applies is $500.