Here is the reply I got from GetUp:
It’s true that councillors can’t simply cancel a development because of a petition. Planning decisions still have to go through a formal approval process and be assessed against planning rules.
But that doesn’t mean petitions are useless.
What petitions do is show the scale of community concern before and during that process. When councillors, planners and governments see that thousands of people are paying attention, it often leads to much closer scrutiny of a proposal and can influence how decisions are made, what conditions are applied, or whether projects move forward at all.
Community pressure has played a role in many planning debates in Australia. Petitions are one way people can make sure their voices are part of that conversation.
So the honest answer is: a petition won’t decide the project on its own, but it helps demonstrate that the community cares and wants decision-makers to take a careful look. We're also considering delivering the petition directly to the mayor – we've got over 15,000 signatures already and climbing, we're sure this will show the community doesn't want the tower to go ahead.

Rose petals? Have you heard of sarcasm?