it looks like 150mm x 150mm cypress pine is reasonably straightforward to obtain:
Golden Cypress also looks like it might be suitable, and these guys mill it to whatever size you want
http://stores.shop.ebay.com.au/Add-The-Beauty-Of-Timber/New-Timber-Building-Materials.html ; in Research
also here – http://www.goldencypress.com.au/ ; in Marysville. They have unseasoned!!
These guys look like they add a considerable markup, but might be useful;