According to The Age, the Hunter Valley coal power plant was closed back in 2014. Hunter Energy plans to restart the generator in early 2019. The company understands the demands of cryptocurrency mining, and hopes to make the power plant even more attractive to tech companies by adding cleaner energy sources, such as solar power or batteries.
Cryptocurrency mining is an incredibly power-intensive process. It involves using energy hungry computers to solve complex problems, generating intense amounts of heat and using quite a bit of electricity. As a result, miners and mining companies have been on the hunt for inexpensive electricity. Operating from within a coal plant meets that requirement for sure.
The problem here is that coal-fired electricity isn't exactly the cleanest source of power. One of the main arguments against cryptocurrency mining, and a reason why countries like China are seeking to ban it, is that the energy required generates a lot of pollution. Bringing a dirty, pollution spewing power plant back online specifically for cryptocurrency mining doesn't seem like the best or most thoughtful long-term solution to this issue, even if the company is exploring plans for greener energy in the future.