I fly this route to MEL very very often, never had any issue in Sydney making the connection to/from MEL. Everyone has to deboard, there is no staying on the plane.

There's a groovy hotdog kiosk in the AirNZ (UA) lounge in Sydney which is worth saving room for. UA lounge in MEL is very bland in comparison, try the AirNZ lounge on lower level if you have your *G card with.