Did a set of flights last year when my daughter was 15 months old. CBR-MEL-AKL-RAR-AKL-MEL-CBR... On each flight bar the first CBR-MEL flight, my daughter was given her own seat, even though she was only travelling on an infant fare.
Both the ground staff and FA's mentioned to me that whilst they don't block a seat off specifically for her, they will make sure that seats next to parents with infants are some of the last to be assigned.
There was a chance that she wouldn't have a seat for AKL-MEL, however shortly after take-off the FA's invited the single traveller sitting next to us to move into another seat, which he took, and thus again gave us the spare seat for our daughter.