Orders build from 7:00am. Around 9:59am one auction sets every stock's open price, then continuous trading starts.
From 7:00am Sydney the market is in pre-open. You (via your broker) can enter, amend or cancel orders. Nothing matches yet - the book just fills up.
Around 9:59am the Opening Single Price Auction (OSPA) runs. Overlapping buys and sells are matched at one single price chosen to maximise matched volume. That price is the stock's open for the day.
During the auction itself you cannot enter or amend orders. You can cancel existing ones. Permitted trade reports can still be lodged. After a short buffer, continuous matching starts (earliest about 9:59:45am).
The two thin striped bands are random 15-second windows. ASX picks the exact switch time inside each window so nobody can game the millisecond the auction or the open begins.
Since 23 June 2025 this applies to all ASX cash equities together - the old A-Z staggered open (roughly 10:00 to 10:09) is gone. Times are Sydney. The close has its own auction later - see closing auction.