Wilma85, they are filled in the order its placed. However, if say you are 5th, and there is 4 orders in front of you with wespac, and the selling is commsec, your order will get filled as XT.
Happened to me yesterday at 10c, I was 8th down the list and got 110k filled.
615 100 shares got filled at 10c yesterday (110k is mine)
326 201 shares got filled today as of 1:15pm (190k is mine)