ThaiExpress would have to be the most authentic Thai food that I have found in Sydney for some time.
Ironically, they are a Singapore based Fast Food chain, but they are certainly passionate about authentic Thai cuisine. All kitchen and wait staff are Thai in the Sydney store.
On my second visit, I had the Vegetarian Green Curry Laksa (pictured) which was simply full of flavours and all vegetables were crisp and perfectly cooked. AUD$12.90