Ann: Appendix 2A - Option Conversion, page-16

That's the opening auction. Trades start in staggered lots alphabetically by ASX code.

ASX Open

Opening starts at 10:00 am (Sydney) and lasts for approximately 10 minutes.

Group 1 -10:00:00 am Letters A-B (e.g. ANZ)

Group 2 -10:02:15 am Letters C-F (e.g. CBA)

Group 3 -10:04:30 am Letters G-M (e.g. GNC)

Group 4 -10:06:45 am Letters N-R (e.g. NAB)

Group 5 -10:09:00 am Letters S-Z (e.g. SEK)

There's also a closing auction, but all ASX codes finalise the auction at 10 minutes past the ASX end of normal trading (usually 4:10 PM).

You can add and remove offers and bids at will during the auction.

Both are often used by traders trying to buy for a pip less or sell for a pip more. Involves some luck

Trading Hours (ASX & Global) - Market Index