I am generally opposed to fencing off car spaces and adding a fence to this situation isn’t going to do much top make your life easier. Honestly, I would go back to her and say, something like:
Forget the fence, please just get rid of your boxes and stop using visitor parking for your own purposes.
The boxes are unsightly and a fire risk and the garage is specifically designated on the plans for parking cars (if it is). Using it as a store room may be in violation of our strata insurance (worth checking).
Visitor parking is for visitors and, according to strata experts on flatchat.com.au, using the space permanently for your own purposes is a form of theft. I’m sure that ws never your intention but now that you know that, it’s up to you to do the right thing – and that means moving your accumulated jumble from the car space, as a first step.
OK, this could start World War 3 but you have rights and in this situation, she has none.