This is from an email I got from them when I encountered a similar scenario a few months ago:
"The offsetting policy (contra) follows the following guidelines:
A purchase will be offset against the proceeds of sales executed on the same trading day and the following trading day. (i.e. Buy 1st then Sell (T+1)
The proceeds of a sale will be used to offset against any purchase executed on the same trading day, or the following two trading days. (i.e.Sell 1st then Buy (T+2)).
Any outstanding balance remaining for purchases and sales of shares after offsetting will be automatically settled through your nominated bank account on the third trading day.
[...]
You can compare the contra policy outlined above with your own account financial history by logging into your CommSec account, selecting My Portfolio and then Financials."