Aussie here, We're still very much a car centric country, but for major public events we are pretty much hardwired to use public transport. I dont know what the actual parking capacity of the MCG is, but Marvel Stadium (Docklands) is only 500 cars with 55000 seats(not including the many nearby free and paid parking locations), that is much smaller than the MCG at over 100000 capacity, one of the worlds highest capacity stadiums despite our puny population.
Apart from most stadiums having extensive train, bus and drop-off access, we also have Park and Ride programs that can temporarily scale up to cover events like this which can basically turn any unused land into a large temporary carpark with dedicated bus services that take you either to the venue directly or to the local train station when they both have limited parking.
Soulseek has been getting hammered too