I have had to work in Perth and the shopping is absolutely dire, so I really sympathise. Assuming you're confined to CBD, for cheaper stuff that you can dump later, start with Portmans, they have half decent work basics that you can dress up. Trousers are particuarly good but the jackets aren't well tailored, they are built to a price. But really worth checking in there.
Forget Cue - it's useless for anyone over a D cup, and it's poor value for money.
Ojay is much better quality, but don't buy from the main store until you have checked the outlet in Harbourtown first - they often have the same stock as the main stores, but it's much cheaper at the outlet. Really irritates me that they do this so check the outlets on a Friday night if poss when it's quieter. Ditto Oroton at Harbourtown.
have a good look at all the different little concessions stores inside Myers & David Jones - I found some gorgeous workwear in little concessions that didn't have full stores. Avoid Sussans/Brown Sugar et al, the stuff is overpriced polyester and so badly tailored. Horrible. Keep an eye on Laura Ashley, they have some lovely things from time to time, but you have to be selective.
Agree a decent bag/shoes/scarf really works, esp in label concious place like Perth, if you've got an expensive smart bag no-one will think you're wearing a £7 Dotty Perkins skivvy with that Hermes scarf.
Oh and make sure your nails look fantastic all the time, people do notice.
Finally, don't stress too much about the clothes - Perth is casual, and often the only really stunningly perfectly dressed women you see in the CBD are penniless secretaries who are going millionaire-hunting. Normal women, even CEOs, don't look like they've stepped out of Vogue each day.
Good luck and congrats on the promo!