despite comments to the contrary, Westernport is not a deep port, but a deep entrance to a shallow port, hence the need for dredging. Victoria's only deep water port is Portland, which is all rather small, and with due respect to the residents of Portland, in the middle of nowhere.
Port Phillip has probably reached the limit of dredging without doing catastrophic damage to the Bay foreshore , as the last deepening, small though it was, has effectively destroyed beaches around Portsea , and is having an evident ongoing detrimental effect further up the Peninsula, despite claims to the contrary by the Victorian Channels Authority / VCA ( as successor the Port of Melbourne Authority ).
Also the current Melbourne Container berths are not working at full capacity , which may be a contributing factor to the price hikes.
This also raises the issue of how big can container ships really go. It should be pointed out that currently only Hamburg, Rotterdam and Felixstowe in Europe can take the largest container ships , while Melbourne still cannot easily handle 10,000 TEU ( Twenty foot Equivalent Units ) container ships , as Melbourne is limited to 7,500 TEU, 93,511 Gross tonnage. ( There are both geological and structural limitations effecting Melbourne Port. )
Sydney , Brisbane and Adelaide can all potentially handle the new generation 10,000 TEU ships. As container ships are likely only going to get ever larger . It is therefore quite possible we are now witnessing the decline of Melbourne as the largest container port by volume in Australia .
( The world's largest container ships currently are the Orient Overseas Container Line's "OOCL Hong Kong" class of 6 vessels, at 21,413 TEU, 399.2 metres long, 210,890 Gross tonnes , built 2017, Samsung Heavy Industries , Hong Kong flag. )
Practical restriction on container ship size is also an influence on the increased interest in long distance transcontinental freight railways from China , overland to the west.
As an aside, most of Britain's traditional import docks have closed down due to the geological constraints of enlarging them. Of the remaining traditional UK ports that remain open, their ship size is significantly below the now typical 7,500 TEU container ships.