Your trade is submitted to Westpac's system and then to the ASX...

Your trade is submitted to Westpac's system and then to the ASX system. HFT's would have their own link direct to the ASX in the same way Westpac does so any trade they submitted at exactly the same time as you would reach the ASX before your trade. This also means that the time you submitted your trade would be before it was actually submitted to the ASX, so submitting a trade on your end before a trade going through on the ASX does not mean you should have been ahead of that trade on the ASX. You also have to take into consideration that there is a cross trading system going on before your trade is passed through to the ASX to see if your broken can match another reverse trade.

In terms of seeing your trade before it's submitted to market, I very much doubt that. However they would see your trade immediately and would be able to respond to it immediately (before anyone trading by hand would every come close to doing).