Heng, you’d best contact the wph&s dept here in Qld to get the best advice. They have requirements that must be met to be legal.
You’ll need to conform to the Standards Australia criteria too. To get the books you’ll need to buy them, they cost a fortune though.
For instance, I did a heap of ‘Danger – pool under construction’ signs. My text and layout was OK, but I was informed that I’d used red instead of Black – making them illegal. No one says the laws are clever. I mean, red or black… it says the same thing.