This is an interesting question, and a bit of a legal grey area.
I will preface this by saying - I am not a lawyer.
Let's use an example. If installer A puts a 3kW system on your roof, and then installer B offers you a great deal on adding an extra kW of solar (oversizing your system), which you agree to, who is responsible if something goes wrong with the system?
Well - first and foremost, adding or changing an existing solar system means it must be made compliant with current Australian standards (meaning the non-compliant panels or inverter would need to be totally replaced) - and if your system is more than a few years old, there's a high chance your system no longer meets the current standards.
(Note that this doesn't apply to repairing solar systems - if you're simply swapping out a broken component with a like-for-like replacement you don't need to bring it up to current standards)
For this reason, 9 times out of 10, installer B will simply install a new, separate, system that sits on the roof next to installer A's system but is totally independent of this. This is where microinverters are usually deployed to great effect.
But - if the existing system is still compliant with Australian standards and installer B chooses to use the existing inverter and wiring to add to the system, what happens next with the warranty?
Some solar companies have a clause in their warranty agreements that says something along the lines of "If a third party (another solar company) modifies the system, the warranty is void".
I'd argue that this would be found unreasonable under Australian Consumer Law - but again, I'm not a lawyer.
If installer B adds panels and designs it all within the specifications of the original system, then my opinion would be that if anything of installer A's system failed, then they (installer A) would have to honor their warranty. But it may be one hell of a fight - and the decision would rest with a magistrate if they had to take it to tribunal.
I personally think it would be unfair to make installer B responsible for installer A's system if a part of that goes wrong.
So - as mentioned above - if you want to increase the amount of solar on your roof, your best bet would be to simply ask installer B to install a totally separate system next to installer A's, defusing any potential warranty fights.