It’s not as dangerous as it sounds. Solar guerrillas use commercial mains power inverters which meet the relevant international standards to feed their electricity into the grid. You plug your solar array into the device, and plug its output lead into a normal powerpoint. It’s that simple.
The inverters used have “anti-islanding” features, where they shut down if the mains power drops below a minimum level. This is an important safety feature to ensure you don’t hurt linesmen or others who may have disconnected the power to carry out work.
Most of the time these solar systems are quite small – often just 100-200 Watts (outputting maybe 1 kWh/day if you’re lucky). If you have an old-style electricity meter with the spinning disc, your solar array will run the meter backwards (thereby reducing your bill) whenever it is generating more electricity than your home is using. Newer electronic meters prevent this.
It needs to be said that guerrilla solar installations are generally illegal in most states of Australia (it sounds like you might be able to do it in Victoria as long as you notify the appropriate authorities?). Still, the technology and engineering behind it fascinates me. It’s quite safe as long as the inverters used meet certain standards; their banning seems purely political, no doubt driven by the large energy utilities to benefit their own businesses.
Some further reading on the topic includes: