FEX Valuation, page-699

Panamax size ships, the size FEX uses, carry 80,000 tonnes fully loaded.
In the case of Geraldton, depth constraints in the harbour and the channel limit the 'full load' to about 60,000 tonnes.
The dredging being done is to remove accumulated sand and small pebbles that have washed into the channel and the harbour area over the last ten years. The dredging is designed to restore the status quo, not to increase the depths. I.e. it is maintenance dredging.
Panamax size ships leaving port are sometimes further limited when the swell is up. As the ships leave port they start to roll in the swell. A rolling ship on the "down side" draws more than a ship sitting on its lines. Somewhere, on the Midwest Ports website, I think, there is a table that details how much IO must be left off the load to allow for the sea state.
You can have as many trailers on the road trains as you like, you still won't fit more than [about] 60,000 tonnes onto the ships.