T3 to T2 is just a short walk, assuming the SYD - MEL flight comes out through the one way doors, where the previous T3 screening was.
Of course, the plane might come into T4 zone.
Ask a relo to wait for the older relo, so you can check in together, and can get seats together, OR if you in J, she in Y.
International security screening is first at MEL T2, so passports and all other sundries has to be in bag, some people put passport into the tray, outside of bag but I think security prefers passport inside bag.
Then passport control second.
Some airports in Aus have it the other way around, ie, passport control first, then international screening.