If you want to change to a plan with an equal or higher monthly cost, say to Unlimited Data Plan, you can do so for $0 if you take up a new 12-month contract. If you prefer to change to a plan with an equal or higher monthly cost but without a contract, a charge of $19.99 will apply.
To change to a lower plan, say a slower NBN speed plan or the Superfast plan itself but with 100 GB instead of 500 GB, a charge of $29.99 will apply when you take up a new 12-month contract, and a charge of $39.99 will apply if you prefer to go month-by-month after the change.
If you terminate the plan before the 12-month term ends, a charge of $299 will apply.