Hi Miss-H. I can't help with the legal requirements for WA, but I wanted to encourage you to stick with your guns & hold your dd back. I have taught kindy for years, and have (sadly) failed to convince many parents that it's better to hold a child back than to start them early, and have seen so much evidence that it is better for the child. I have never heard of a parent who has been unhappy about this decision (at both the kindy end of things & the high school end of things), but I have (sadly) heard many parents regret later not holding their child back (but it was too late).
In Scandinavia, chn don't start formal schooling til nearly 8 and they have the absolute best literacy rates in the world. I taught in England for a while where chn generally start 6-12 mo younger than in Australia & it broke my heart. I taught kids who were real strugglers, and it just put them further behind, right from the start.
If it was my child, I'd hold them back, so I just wanted to give you my personal opinion from my experience. I hope this is encouraging for you! Good luck.