Since T+2 was recently introduced, Commsec has been all but useless for quick-turnover day trading.
I suggest you explore opening another account.
I still have a Commsec trading account, but also a Bell Direct account. This however doesn't have a T+x window. Whatever I buy has to be backed either by cash in balance, or a preceding sale of held stock to make the equivalent $ available.
It does at least mean I can sell X now and buy Y immediately afterwards. Something I can't do with Commsec.