As simple as, book for one hour online and stay for up-to 5 hours on the same booking.
As mentioned in the Overstay Pricing for T1,T2,T3 & T4, Melbourne Airport Overstay Pricing overstay pricing is $0 for 0-4 Hours. Which means, 4 Hours + One booked hour.
As it is mentioned in terms and conditions: here
(b) If your vehicle is parked in a car park for any time outside the booking period (i.e. if you leave late) you will be charged for the time outside of the booking period at Melbourne Airport’s Overstay Tariff Car Park rates (NB these rates may vary from the rates quoted via the online booking system) and you will not be entitled to a refund or credit on those charges. Click here for Melbourne Airport Overstay Tariff Car Parks Rates.
Haven't tried it though,
May be i have miss-understood the policy.
Comments?