When Rivals was being built, containers would arrive, be unpacked, and constructed, and then more parts would arrive.
By all photos, a significant block of Dreamworld's carpark is already fenced off to house parts that have arrived, or will already be in transit.
Until such time as they commence construction (and assuming the ones that have arrived would have been the first ones constructed), holding back some of the last pieces until they know when construction will commence is probably a smart move - otherwise, they could well end up with the entire carpark filled with coaster parts...
Remember, as far as we know, the footings haven't been drilled yet, and the collars are still in the carpark, so we're a ways off vertical construction yet - maybe that 30 day voyage time isn't such a big deal right now...