The Residential Tenancies Bond Authority (RTBA) does not charge a fee to change names on the bond.
The law does not prevent the real estate agency from charging an administrative fee. If an agency incurs administration costs or charges for providing this service, it may pass the cost to you.
You can ask for a breakdown of costs from your agent. If you feel the fee is excessive or unsubstantiated, you can apply to the Victorian Civil and Administrative Tribunal (VCAT) to contest the fee.