The link I sent was to a KNAUf website. , and among many products they sell is the XPS board in several sizes. A quick phone call to the phone number in the top right hand corner of the website opens up several options.
One was Bunnings as somebody else mentioned.
They can order products off range or check to see if a store my have stocks of the larger sheet 1200 x 2200 which has been removed from the regular Bunnings range.
The helpful person on the line also advised me of a company who stock the larger sheets and does not charge 100 bucks a delivery.
As nobody else has offered any on the ground experience in Sydney it may explain why Bunnings is no longer stocking the larger sheets.
( possibly lack of demand).
Another approach may be to check out if any friends, colleagues, nearby modellers would be interested in buying a large pack and splitting it to minimise the delivery charge. A collaborative approach may even unearth a way to obtain/ share/ hire a cutting tool etc and maybe allow a share of information, experiences and even an extra pair of hands to help you assemble your module(s).
Cheers and happy modelling.