Burger King have 2 single use (per account) coupons in their app. BOGOF Hershey's Chocolate Pie ending on 24th June. BOGO1/2P BK Chicken BLT ending 30th June. The prices in the title are from a store near me, yours could vary.
If you don't like bacon or tomato you could always remove it and get a regular BK Chicken although you'll only be saving about $1.85 for the 2 burgers then. (And they had a $5 BK Chicken deal a few days ago.) Be warned that the BK app is known to be problematic with some stores.
These aren't valid at Christchurch airport as always.