Firstly, double check the coversheet you got from Dial Before You Dig: does it include any water authorities on that list?
If it does, then contact them and ask what is going on.
If it doesn’t, then it is likely to be because the water authority in that particular area has not subscribed to Dial Before You Dig. It is currently not mandatory in Victoria to join Dial Before You Dig so there are some asset holders (mainly water authorities and councils, although Vic Roads is another one as well) that have not signed up to it.
As such, you will need to contact the water authority directly and ask them to send you out plans for that area.
It is a real pain, let me know if you need any help with it.