I posted this here which is a good deal for the physical copy. However if you are tired of having to swap discs everytime to play a different game here is something to think about.
If you buy Super Mario Kart from the eShop for $10.40 by 30/06/2014 you will get $10.40 off MK8 ($79.95 rrp) when it is released on 31/05/2014.
You might think this is a raw deal however if you have a Wii U Premium pack don't forget you are enrolled in Nintendo Network Premium which will net you 534 points for your SMK and MK8 purchases. 500 points = $7 credit on the eShop so you are paying $79.95 for SMK, MK8 and a $7 eShop credit.
Might not be as good a deal if you don't want both games but if you have a Wii U Premium and intending to pay the $79.95 retail I would strongly suggest you buy from the eShop.
EXTRA BONUS
If you pay for eShop purchases using Citi Platinum/Signature Credit Card you will earn 3-4 points per dollar (depending on card) since Citibank thinks it is an overseas transaction (lists the transaction as from 'Gb' not 'Au'). Nintendo Australia probably "skins" the EU eShop, which is why when you purchase from the eShop there is a notice saying 'your credit card provider may charge you a fee for overseas transaction'. However this is not the case :)
Not terrible and I do appreciate the effort.
Its not terrible if you want Super Mario Kart but lack luster otherwise.