Hi Hadyn,
Has the project ended and therefore is your contract now coming to an end? If so, then your company may be able to offer you a lower rate of pay (as long as it is still the minimum rate under the applicable Award/ Enterprise Agreement) under a wholly new contract. Of course though, you aren't under an obligation to accept.
If the project is still running and therefore your contract hasn't ended, then as I said earlier, the company has limited rights to change the terms of your agreement. As I said before, you should definitely contact Fair Work Australia if this is the situation.