Got some codes from Letterbox Deals in the mail but then I did a Google search and found a code with a longer expiry date via Groupon. Codes are for new customers (can't fake your plates).
- CF1FN4 for 1 Day Free Parking (e.g. 24 hours or less). Expires Dec 31
- LBPWJ28 for 2 Days Free Parking (5 Days Min Stay) Expires Dec 31 (New code)
Screenshot. Code appears to work for valet, undercover or outdoor self parking.
T&Cs:
Bookings must be made online. Valid for one per customer. Not valid on/over public holidays or school holidays. For travel completed by 31 December/30th June 2018. Not to be used in conjunction with any other offer. Subject to availability. Not valid for valet parking option.
Still more expensive than the regular Melbourne airport long-term carpark for me even after use of discount code.