thing is I don't want to turn her. esp. before 6mths, it was going on about how you can use it to rearface until 4 years! so thats why I"m really stumped as to how a 5mth old is too big. makes me think Im doing something wrong, which then makes me worry because that might mean she is unsafe.

You can use the seat until they are 4, not with it rear-facing though.
There aren't any seats approved for use in Australia that go beyond the 9kg/6months/height restrictions, this is the problem. The height and weight are the key factors though with regards to the seat and turning them, the age is really irrelevant, as the seat is tested for weight and height.

Call Britax and ask them directly to clarify for you exactly what the marker is telling you.