Like the other girls have said, get maternity bras as soon as your normal bras are uncomfy. I second getting professionally fitted too as they are a little different to normal bras (as they are designed to grow with you a bit) and it's not as simple as getting the next size up.

I've had the most success with David Jones and they often have sales now to compete with Myer it seems so you can sometimes snap up a bargain. I'd recommend Lovable bras but I'm not sure of their maximum sizes so you may need to go to a proper lingerie shop. My SIL has E-F cup boobs while BF and she struggled to find anything to fit her at the usual department stores.

Enjoy wearing them when you find some, they're super comfy!