My 2 year plan ends in April 2019. Is it possible for me to complete this earlier without having to pay the complete cost of the plan?
Sadly no, you agreed to the 2 year term of plan.
You would only pay out the remaining months not the full 2 years though.
Most Optus plans have a set exit fee. This fee is prorated down the longer you are subscribed. So for example a $300 exit fee would now be just $35 dollars to wind up the contract. Use LIVECHAT and ask Optus what the exit fee would be for your particular plan. Its likely not much.
Note that and monthly handset fees will still need to be paid out in full.