When the mill was constructed, the dominant form of motive power for industry was steam power. In most places in Australia the use of water powered mills was impractical due to lack of good flowing rivers.
Milling is a energy intensive process, so it is understandable that over the years there were upgrades and issues with making sure the mill was kept running smoothly. there are some accounts of the mill operating 24 hours a day in busy times, so having enough reliable power was essential.
1865 Startup
The initial engine used when the mill was commissioned was a 10HP portable steam engine with double cylinders, manufactured by Ransom and Sons, Ipswich (England). These portable engines had large steel wheels so they could be towed by horses to where they were needed. (https://en.wikipedia.org/wiki/Portable_engine). Later they were engineered to be self movable and were known as traction engines. The portable engines are a fully integrated unit with firebox, boiler, chimney etc in the one unit. Possibly this was employed here at the mill as an interim power source - most early images of the mill show a large chimney which would not be required for a portable steam engine.
A 60 ft well on the property was supplying the necessary water with the steam engine also supplying power to a well pump.
This note from the Ovens and Murray Advertiser (1868, August 29) confirms the presence of the original steam engine.
https://trove.nla.gov.au/newspaper/article/197438490?searchTerm=tarrawingee%20flour%20mill
The above article says the manufacturer was Ransome and Sims. This company had several names over its life, including both Ransome & Sons, Ransome and Sims and Ransomes, Sims and Jefferies (https://en.wikipedia.org/wiki/Ransomes,_Sims_%26_Jefferies)
1868 Steam Engine Upgrade
In 1868 there are advertisements where the mill owner is selling the initial 10HP portable steam engine, and later in 1869 an article about the new 25HP engine having been fitted. Details from the later sale of the mill (1870) say the steam engine was 30HP with substantial dimensions - engine 16¼ x 3 feet cylinder, with expansion gear by Tennant, Boiler, 25ft 3in x 6ft by McCall, Black & Co., with cross tubes. These are substantial dimensions and would have been incorporated into a brick structure which included the large chimney.
1882 New Boiler
In 1882 the mill was put to tender for lease and part of the tender notes was that a new boiler was being put in during the month of December. There are not many details of this, but presumably this would have been a major project. This was the first of several boiler issues - the problems were ascribed to the corrosive nature of the brackish (salty) water from the well.
1889 Cracked Boiler
In May 1889 the owner noticed steam escaping from a crack in the boiler and evacuated the site. Rather than make repairs, the owner ordered a new boiler. This put the mill out of action for two weeks, suggesting that the boiler was available locally, probably from Melbourne, but possibly still of overseas manfucture.
1908 More Boiler Problems
During routine inspections, a "weak patch" was observed in the boiler, and that a 6ft x 2ft plate would need to be used to repair it. A boilermaker from the "metropolis" (presumably Melbourne) and the local smithy, George Black, were engaged make repairs. This appears to have been successful, but the days of the boiler were numbered.
1909 Moving away from Steam Power
Possibly as a result of ongoing issues with boiler tank integrity, around this time a decision was made to move away from steam At that time the emerging technology for heavy machine engines was gas engines, although soon both gas and steam engines were soon to decline in use in favour of electric engines. These "suction gas engines" were much more powerful and more efficient - they could run on three bags of charcoal being now used, in place of 1½ cords (3m3) of firewood as formerly. Because there was no supply of town gas, a gas generator was required to burn the charcoal to generate gas for th engine. This was essentially an internal combustion engine.
This system was a 50HP engine manufactured by the celebrated firm of Messrs. Hornsby and Sons. At the time it cost £1000 to install, which was a sizable sum (about A$150,000 in 2021 money).
What happened to Gas Engines?
At the time the mill converted to gas engine, steam was already in decline as the source of motive power for industry, and it seemed that gas engines were the future.
Source: 3rd Australasian Engineering Heritage Conference 20093rd Australasian Engineering Heritage Conference 2009Gas Engines in Victorian Industry, 1870-1950M.S. CHURCHWARD, BE, MEngScSenior Curator, Engineering & Transport, Museum Victoria
However soon after the mill upgrade to gas, both steam as gas would start to decline in favour of electric engines. However these engine were highly reliable, and there are now reports of any engine related issues from installation until the mill destruction in 1918.