As some people choose to travel again it could be a timely reminder of this great deal, although it has changed slightly since.
It's now $8 for up to 3.5 hours parking in T123.
For those unfamiliar: you book for 30 mins but with the grace periods either side you can extend that to 3.5 hours without extra charge. For example, you plan to be at the airport from 3:30pm to 7pm. You book for 5:30pm to 6pm and pay $8. With this booking you can enter any time after 3:30 pm and you have to leave before 7pm to avoid extra charges. The drive up rate would be $49 and if you booked for the 3.5 hours you intend to use it would be $34.
Snippets from the FAQ:
Can I enter the car park earlier or later than my confirmed booking time?
… You may arrive up to two hours before your booked entry time … There is a one hour grace period for exit after your booked exit time …
Update 1: As JaaayD pointed out the T&C’s are worded differently, I suspect they haven’t been updated. Buyer beware.
Update 2: From fedex11’s comment the FAQ appear to be correct and you can enter up to 2 hours before without additional charge.
T4 is now a flat rate of $12 so not a great option for the online booking hack as the cheapest rate at T123 is cheaper. Also the names have changed, it's now Terminal, Premium and Value instead of Short Stay, Valet and Long Term.
I booked this yesterday. You swipe in and out with the credit card you booked with (actual physical credit card, not Apple or Google Pay etc). No issues with me staying for longer than the 30 mins booked. I only stayed for about 50 minutes so I didn’t push the limits.
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Update (15/08/23): Entry and exit is now done via a QR code that’s emailed to the booker. Thanks to triplea.
update from my end. went on last month, I was out in 30 mins. I didn't get to test it fully.