What happens when a stock is added to ASX300, page-6

"rebalance isn't until September" - The March rebalance was deferred due to excessive volatility. It is now planned for June 22, 2020.
Go here and scroll down, looking at the left side of the screen:
https://au.spindices.com/indices/equity/sp-asx-300/#overview
You will see this:

Clicking it will open a PDF that has this:
Effective Date June 22, 2020

Except for the indices in the categories above, S&P DJI will postpone the index rebalancings that werescheduled to rebalance in March until June. The March quarterly share and IWF updates will also bepostponed until the next scheduled quarterly rebalancing in June.

So rebalance is less than two weeks away.