As title states, Target branded Double Bed Mattress Protector for $2.86, down from $20. Also available QB at around $8.50. There were about 10 of each size at Target Chadstone.
Target seems to have a centralised pricing system, therefore this may be available else where. YMMV.
EDIT: Seems like the availability is quite low, none at Target Burke St, and Tam reported none at Fountaingate.
for 'fitted' this is an excellent bargain!