The OCV are also to replace some minor vessels like the mine hunters and much older survey ships. I would imagine they would replace our patrol boats eventually as well. However the first ships may work in combination with the patrol boats, because the patrol boats frankly aren't big enough to deal with migrant boats arriving in our north. There is nowhere to put 100 people, even for very short journeys or stays. It is rather silly to use frigates for this purpose as they aren't designed for it either.
They may finish the first build in ~5 years. Build rate could happen slowly at 1 per year, or much faster if required.
It should be designed to take up to 76mm, but most of them would be fitted with typhoons. Provide spots for seaRAM and/or Phalanx which can be fitted from the pool. A mine hunter or survey ship doesn't need a 76mm or Searam most (any ?) of the time. The LCS 57mm gun seems to be attractive, I'm sure the RAN would be interested in it for the few ships going into higher risk areas. Other LCS systems/weapons may also be fitted. But they aren't front line warships or even corvettes.
These ships would be perfect for EEZ policing, people smuggling and allowing Australia to patrol international waters when this Japan verse sea shepards thing goes ballistic, or simular incidents. (see cod wars etc).
They are not going to be armed to the teeth (although in war time they could be upgunned, there could be space avalible for harpoons, Helicopters, 76mm, UUV's, Searam etc). But are designed to be a sizeable vessel with good "peacetime" capabilities. Rescues, searching, patrolling, enforcing, boarding, policing, surveying, delivering aid etc.
Freeing up the 11-12 7,000t ship fleet for high intensity stuff. These ships could also work under the protective umbrella of the frigates or AWD to enforce in higher threat enviroments like ME or Asia. So that you don't end up in the situation where inflatable boat boarding parties get cornered by patrol boats with help very far away.