Trades can be conducted after hours if the two parties (usually banks or brokers) are in contact and agree to a price. I dabble in algo trading (Not HFT, calm down, it's a portfolio diversification algorithm....) and the closing price for the day can and does change based on these after hours trades. I checked specifically for this possibility so that I would know whether I had to re-fetch closing price data to make sure it was up to date. It would've made my life a lot easier if the 4:10pm price was final
Disclaimer: This algorithm runs on overseas exchanges so I cannot confirm this is 100% accurate for ASX, but i'm pretty sure it is
