It is possible (not pre-paid though). Optus Small Business accounts allow sharing of data between all devices on the account. So, 2 iPhones with 3Gb each, and 1 iPad with 10Gb = 16Gb total which can be used in any way over any of the devices.
Optus will also "sell" you an iPad on a plan - just like a phone. Convenient for business (YMMV, this is not advice), as the monthly account is tax deductible straight off, without having to worry about depreciating the device. Also saves on having to pay up-front for the iPad.