Aren't I legally required to lodge a bond in Victoria?

No. In Victoria, there are three options available to a tenant, property manager and landlord. You can pay no bond by mutual agreement, you can pay a bond via the RTBA, or you can provide a bond guarantee as outlined in Section 37-38 of the Residential Tenancies Act 1997. Bondable meets the requirements of Section 37-38 of the Residential Tenancies Act 1997.

You will only need to lodge a bond if a claim that is referred to mediation exceeds the value of the bond. See here for more detail.