daytrade diaries... october 26, page-31

This may help some,

Opening Phase
Opening takes place at 10:00 am Sydney time and lasts for about 10 minutes. ITS calculates opening prices during this phase. Securities open in five groups, according to the starting letter of their ASX code:

Group 1 10:00:00 am +/- 15 secs A-B, e.g. ANZ, BHP
Group 2 10:02:15 am +/- 15 secs C-F, e.g. CPU, DVT, ECP, FXJ
Group 3 10:04:30 am +/- 15 secs G-M, e.g. GHP, KYC, LIB
Group 4 10:06:45 am +/- 15 secs N-R, e.g. QAN
Group 5 10:09:00 am +/- 15 secs S-Z, e.g. TLS

The time is randomly generated by ITS and occurs up to 15 seconds on either side of the times given above, e.g. group 1 may open at any time between 9:59:45 am and 10:00:15 am.