I'm from Sydney so this is my take on things. Have lived in Brisbane and have visited Melbourne a number of times. Never been to Perth so can't comment much.

Sydney:
pros: nice climate, great beaches, great Asian food. good public transport for the most part
cons: housing is expensive as fuck, hustle culture, hard to break in and make friends, alienating.

Melbourne
pros: great cultural scene which is very accessible. easy to just walk around and find cool shit happening. food is good. generally a cool vibe to the city. public transport is great in the inner suburbs but gets patchy when you go further out.
cons: gets fucking cold. "4seasonsinaday", no good beaches.

Brisbane
pros: laid back vibe, cheaper housing, pretty livable, easy to get around mostly, weather is beautiful in winter months.
cons: boring (imo), too hot for like 9 months of the year (imo), more bogans.

Perth
pros: not sure, probably beautiful location and beaches but never been.
cons: not sure, its far away from every other city, small population, i hear its kinda boring but never been so cant really comment too much.

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>>2876825
Pros: hot chicks, nice weather
Cons: SPOIDERS

Applies to all four cities