Ann: S&P DJI Announces March 2023 Quarterly Rebalance, page-6

It's a positive in the short term because index funds such as the $30bill Vanguard Australian Share fund needs to buy whatever the index weight will be (most index funds track the 200, vanguard tracks the 300). My guess, looking at the current exposures of 400-500m market cap companies such as aef, adh, shv, that are currently in the index, is that the weight is around 0.02% (there are other factors apart from market cap such as free float), which is about $6m worth or around 23m shares that needs to be bought.

then there are index funds which track the small ordinaries (companies in the 300 but not in the 100) but not all of these funds employ a "full replication" strategy so the small ordinaries buying will be small or negligible.

however once the bout of short term buying ends, the shares held be index funds are generally lent out to whoever wants to short which eventually negates the initial buying.