Unfortunately, the general rule is you can't drop your plan value.
If you are paying for extra speed you can usually remove that for free ($20 for 100Mbps => 25Mbps and another $20 for 25Mbps => 12Mbps)
You can also remove optional telephone options ($10 removes Free national, mobile calls another $10 for international calls)
How much are you paying each month for your plan? Do you have a Optus Fetch PVR? What is included in your plan?