I highly doubt that comsec would run trades against their own clients in such a fashion. If the ACCC got wind of it at all, they'd be big bucks to pay.
Anyway if you suspect that during the confirmation that you're being frontrunned by comsec, place a false order in for 1 million shares of the stock you want to go up, watch the price spike, don't actually buy it?
Disclaimer: I am in no way associated with comsec or employed, by commonwealth bank.