Barnet, probably the easiest/cheapest way to short-sell is to use CFDs (contracts for difference) through a provider like IG Markets or CMC who allow you to trade in both directions. You would need to open an account with one of them - easily done and there is no conflict with also having a CommSec account.
However, CFD providers offer serious leverage, so traders need to be very responsible with their position sizing because wrong bets can blow out very quickly into heavy losses. It's a risky endeavour that has blown many a novice trader out of the water. Good luck with it if you go ahead.
