Is this scenario doable?

[OATEK]

Round 2! 1h30m from the City to the Airport OLCI + Carry on possible? I can't leave earlier than 4 and I'm hoping to get QF5 departing at 5:25pm opinions? too risky?

I think you know the answer - it can be done, but not much wriggle room, especially if you are going through the general security/immigration channel. OLCI beats the 90/60min check-in closure, carry-on only all good. But train/walk will be 20mins+ then security, immigration... Be interested to hear how it goes, and hope it goes well for you!