Have a friendly chat with your neighbour about how you can work together to control weeds. If neighbourly negotiation is not possible, Agriculture Victoria are the authorised body under the CaLP Act for enforcement action on landholders who fail to meet their legal obligation to control declared weeds and pest animals.
Please note, local government has no enforcement powers with respect to noxious weed control on private property, however if a property is dangerous or unsightly you can report this for Council to investigate.