I think the iress open takes the close price and the group one stocks dictate the open price at 10am this includes all the pre open match of BHP.
Group 1 10:00:00 am +/- 15 secs 0-9 and A-B, e.g. ANZ, BHP
You are correct it probably is not as important for an index to have an open gap as it is for an individual stock - So bash the candlesticks you reckon
I have tried to get my head around your wykoff stuff but I must think with the left hand side of my brain or something - Mrs h00ts reckons I think with some thing else a bit closer to the ground