In a lane off Little Collins Street, deep underground, is the Russell Place electrical substation. It is a particularly notable place because it has a working mercury arc rectifier, the last in Melbourne I believe. The substation was built in 1882, making it one of Australia's earliest.
When I say working, it doesn't actually supply power any more but has been kept operational for historical display purposes. So what does it do? Or did?
I don't know much about electricity. Here is what I think is right. AC, alternating current, is the type that comes out of your power point. This was not always the case. Maybe not in Australia, but some homes were powered with DC, direct current. I don't know why DC was preferred over AC, but it was, especially for powering lifts, trams and trains, machinery such as a wool press, pumps and other heavy equipment.
Electric substations for Melbourne trams were sprinkled around the city. They converted 240V AC to 600V DC. Some of the substations used mercury arc rectifiers to make the conversion. They were like a giant blue lit light bulb and contained mercury. As the power required waxed and waned, the lamps grew brighter or duller. The more power being drawn, the harder the rectifiers worked and the brighter they glowed.
If you have been around for a while, you might have ridden on an old W class tram and when they were out near their suburban termini and the power supply was low, you could see the interior lights get brighter or duller as the driver cut on or cut off power or other trams in the vicinity did the same. Back at the substation, the rectifier would have been doing the opposite to the tram lighting.
Now this might all sound very much old stuff, but it isn't that old. The mercury arc rectifier in Russell Place was still working in 2003 supplying DC current to the lifts in the nearby Victoria Hotel in Little Collins Street.
Here is the substation, such an innocuous looking building, but below ground level, it is massive with several levels.
Below is a Youtube clip by JennaDDDD. She? states it supplied 750V DC, but I think it was 600V DC for trams at least. I am sure I have seen one of these in the Tardis. Maybe Tardis is powered by 600V DC.