I drive past this structure quite often and didn't know what it was so I googled it and this is what I found out.
An extract from NORTH QUEENSLAND'S MINING HERITAGE TRAILS
"At the entrance to Mount Molloy township stands a boiler and steam engine, part of a timber mill constructed in1904 by the Mount Molloy Copper Mining Company. The brick boiler mounting contains an intact Stirling boiler alongside a rare Marshall centre-flywheel compound engine. The mining company survived on the timber trade after lack of ore placed its future in doubt. In 1914, the mining company went into liquidation and all operations closed down"