I saw the big sell order today and decided to buy at the...

I saw the big sell order today and decided to buy at the opening, but created and order that would not get completely executed, so I could buy the remaining of my order at .275 to bring the price up, since there was a big gap in the depths this morning. Bloody Commsec did not allow my order to fill at .275 when price was .235. They say I cannot buy at a price much higher than the last negotiated price, but someone is allowed to sell at a huge discount on the opening. These rules do not make any sense. If I had bought at .275, the book would readjust at higher level, not the current trading range. Why I am not allowed to protect my portfolio if the only selling offer available after the opening was 0.275?