The best spa bath surface for swimming spas is a concrete slab that will not move.
If the base can move then your warranty will be void and the swim spa could crack due to movement and stress to the shell.
In general, for swim hot tubs the base must be able to support 1000kg per square metre and this requires a level slab that is 50mm (6”) minimum thickness with F72 reinforcing mesh.
You should consult an outdoor spa installation specialist if you are unsure of your soil type. Contact Oasis Spa store Australia for more advice and information.