Girls and boys changing together in year 5/6 would be a problem. However in the Sydney primary schools I am familiar with it is not an issue as on PE or sports days the children wear sports uniforms all day (T-shirts, shorts, and maybe tracksuits). The only place a whole class could possibly change would be the classroom or the open air.
I have no memory of changing with girls at primary school so maybe the same applied. However, I do remember one very embarrassing event. At age 10/11 I was told to strip to underpants in front of the class and stand on a table. I think it was a woman teacher and I was a model for drawing.