20mm xps min for 600 centres. Last jacko job I did, studs were 450mm. Used 12mm boards so did as recommended and added studs at 300mm. Time consuming and added timber cost but solid. Would go for this over 12mm hardie anyday.
xps boards here also. Easier to work with and fully water tight if installed properly. HB has it's place. For me.. that is over boarding a wooden floor.