Your spa needs to sit on a flat and level surface. This could be a deck, cobble stones or a concrete pad.
If a deck, it must have been built to comply with Australian building standards, which will almost always be the case if it was built by a reputable builder.
If cobble stones, they must be flat and level and not allow the spa to rock.
If a concrete pad, it should be at least 100mm thick for spas and 150mm for swim spas and ideally extend at least 100mm in each direction from the spa.