Does not work as a straight correlation with the movement...

Does not work as a straight correlation with the movement between US$ and A$.
Just because the A$ has dropped 25% does not necessarily mean US$ costs have dropped 25%.

Imported items such as flocculents would have effectively gone up being US$ based. Conversely depending on shipping contracts could have had a better than 25% drop in costs.

Unfortunately the biggest individual cost item will be the Treatment Charges-they are in US$ from the likes of Nyrstar. They would therefore remain unchanged.

Cheers,TAS