RAN is highly likely to follow the USN. More of a question of when. Harpoon would meets most of the RAN's need in regard to missile strike.
IMO Tomahawk is less important for Australia, its long reach allows it to attack deep land locked targets, from extreme range. However, it is quite an old cruise missile, and not particularly stealthy, smart or dynamic. It would be able to be intercepted by quite old weapons systems and platforms, quite reliably. It can certainly be useful against countries lower on the technology tree, or enmass, or first strike, but really none of these are likely for Australia. Australia operates in a region of archipelagos.
Moving missiles to a dedicate VLS on a submarine is highly advantageous. Carrying mix loads of missiles and torpedo's could require a lot of shuffling in a very tight space very quickly. If your carrying a full warload on a sub, its going to be very, very tight in the torpedo room. Most of the time they are carrying a peace load of ~6, and it feels cramped with 6. I hope that later versions of the attack will have VLS capability.
AFAIK Barracuda has only 4 tubes. So firing off a volley is going to be quite slow.