We kept our daughter back in what was preschool at the time, and now would be prep. So she did a repeat year of pre-school. Her teacher thought socially, she would benefit from it, considering she was a little shy. She said that she would be fine for the first 3 years at school, then when she got to year 4, the curriculum sp?? changed, and got harder and that was when she saw students start to fall behind (she kept an eye on all her students as they progressed through school) If we kept her back, she would develop her confidence more, and be a year older emotionally, and be able to deal a little better (hopefully) with her school work and social situations amongst her peers. If we had sent her on, she would have been going to school with the majority of her peers being a year older than her.

A family member sent their same age child on while we held back. Their child needed tutoring once they hit year 4. Our DD, went into the advanced part of her maths class when she got to year 4. This year, she aced her naplan, and has just been recommended to some committee for extension work at school.

Not sure if any of that will help you or not?
Our DS is born on the last day of the age cut off, and we are already talking about holding him back a year.

In QLD, you can't start half way through the year. (not that i know of anyway)
Goodluck with your decision..