The following link may help explain what is and what isn’t owner responsibility with regard to hot and cold water:
https://www.qld.gov.au/law/housing-and-neighbours/body-corporate/maintenance/utility#
In my experience most Lots in Qld have an independent hot water system. The HWS is generally located under the kitchen bench, in a laundry, or on a balcony. All of these would be owner responsibility to maintain. These should include a tempering valve, particularly in a newly renovated Lot.
Tempering valve information for Qld:
Tempering valves
In hot water storage systems for residential buildings, hot water must be heated to a minimum of 60° Celsius to prevent the growth of bacteria such as Legionella.
A tempering device ensures that hot water is delivered to taps at a maximum 50° Celsius. This temperature is considered sufficient for domestic hygiene purposes and effectively reduces the risk of scalding-related injuries from hot water use.
A tempering device is required to be fitted to all new and replacement hot water systems. They can be identified by a blue, green, orange (solar systems) or black plastic cap fitted as part of the system’s piping.
A quick internet search on the topic of inconsistent flows of hot and cold water reveals this: One of the most common plumbing configurations, is a trunk and branch system. This is where a larger diameter pipe runs from one end of the building to the other, and smaller diameter pipes branch off to supply rooms or individual fixtures. If any of the branches demands water (you flush the toilet), there is less water available to all the other branches.
I am not a plumber but the following information may assist: Installing a new mixing valve in the shower, can reduce or eliminate the temperature fluctuations. Thermostatic mixing valves automatically balance the amount of hot and cold water being mixed, which will prevent drastic fluctuations in shower temperature. If the cold water flow is reduced (due to a toilet flush), the valve automatically adjusts the amount of hot water being mixed. This keeps the shower temperature more consistent, even when other fixtures are using water.
The lot owner could try installing a new thermostatic mixing valve.
Difference between a tempering valve and a thermostatic mixing valve:
* A tempering valve is a 3-way mixing valve that is temperature actuated and is used to temper a heated water supply by mixing the heated water with cold water to provide heated water at a lower temperature, normally 50deg Celsius at one or more outlet fixtures.
* A thermostatic mixing valve (TMV) is a mixing valve in which the temperature of the water from the mixed water outlet is automatically controlled by a thermostatic element or sensor to a preselected temperature that is suitable for direct contact with the skin.
TMV’s are used to prevent scalding which can be caused by incorrect adjustment or inability to manipulate conventional taps. They are available as built in or exposed wall mounted units and have controls for adjusting the temperature and/or the volume of the water passing through the units.
TMV’s are designed to deliver warm or mixed water, to an even, preset temperature and to compensate for any disturbance of pressure and/or temperature on the hot and cold water inlet services.