The first is what some calls spaceship Earth argument: Imagine that you are traveling on a generational spaceship. You have a really good life support system which keeps everyone able to breath, temperature comfortable or bearable and the agriculture pods ticking. It was built eons ago and nobody knows how it really works, but it works mostly. And then one day a bright eyed, young engineering cadet recommends that we tweak a subsystem. Now remember, you have one spaceship and no hangar, no safe harbour where you can dock if things go fubar. Do you let the engineer monkey with the system risking permanent damage?
Earth is like that spaceship. It is vast, and complex, and things affect other things in surprising ways, and we only have one of it.
The second is about international relations and conflicts: Interventions into weather modifications have the potential to be beneficial in one area and disastrous at other areas. You create a deluge here and a desert there kind of things. If Somewhereistan steals the water Otherland's farmers would depend on that's a direct threat to the existence of the Otherland state. Predicting and measuring these changes is challenging. Rectifying harm caused might be impossible even if everyone agrees on what happened, but getting such agreement in a noisy world is hard. This makes weather modification ripe for international conflict. The kind where angry man shoots missiles at each other. Which is better to be avoided.
Australia is "far" from other places, so maybe they are safer in this sense, but memetically they are connected to everyone. Once they proved they can manipulate their clouds who is to tell to Somewhereistan that they can't?