Removal from the All Ordinaries.
https://www.marketindex.com.au/methodology
The constituents are simply the 500 largest companies listed on the ASX by way of market capitalisation. Unlike the S&P/ASX suite of indices, the All Ordinaries has no liquidity requirement.
The All Ordinaries index is rebalanced once a year, on the third Friday of March. One week notice is given to the market with the rebalance announcement being made on the second Friday of March.