I am flying from LAS into LAX with a 90 minutes layover before an international flight to SYD. All flights are with Delta. Is this enough time to get from terminal 2/3 to Tom Bradley, assuming I stay airside?
Yes, that is sufficient time - however only if you stay airside!
Terminals 2/3 at LAX are NOT connected airside to the Tom Bradley International Terminal (AKA TBIT, or simply terminal B). In order to make the connection without having to re-clear security, you will need to catch the air-side bus which departs from near gate 21 in Terminal 2.
The bus runs every 10 minutes, and takes approximately 10 minutes to get to TBIT, where it will drop you airside with no need to clear security.
As both of your flights are on Delta, your luggage will be checked all the way through to Sydney, so you will not need to worry about it in LA.
If you accidently exit the secure area of the airport, then 90 minutes MAY be enough time. The walk from T2 to TBIT is less than 10 minutes, however you will then need to re-clear security in TBIT, which at that time of night can be slow. 90 minutes should give you enough time (especially if you have Pre-check or some way to access the priority line), but it's far more risky than taking the bus.