Currently they would use around ~3MW (megawatts) at peak power...

Currently they would use around ~3MW (megawatts) at peak power and if we go with a doubling of mill capacity it would be around 5MW . Costs of solar panels per MW is around $500K. So I would guess around $600K per MW installed.

They would want to install at least 10MW to 15MW of solar panels so that they can run through the night and on cloudy days. So 6M - 9M AUD for the solar farm.

The more unknown costs would be related to tying this into a system where diesel is available as a backup because it is too cost ineffective to be purely on batteries. Hard to guess these figures but you will be looking at a few million for that. The batteries are costl, probably 1M AUD for 1MW/2MWH, so for 12 hours of storage you are going to need drop $30M on batteries. At this cost you would also be considering other storage options like water pumping and a turbine gen.

However you don't necessarily *NEED* the costly batteries to benefit from the solar, you could just have solar inputting when its available and your diesel generators consume less diesel when this is happening. And if all these millions sound like a lot remember they are spending millions already on diesel per month. So spending $10M AUD up front to save $1-2M AUD per month but not totally removing your diesel need isn't so bad either.