I would be looking at all of her maths work up until this point including previous reports as well as daily class work before this becomes a major concern. Get her to bring her maths workbook home and have a look at what she has been doing all term/year and what kind of errors she might be making and in what area. NAPLAN is hugely flawed as Mrsmac has pointed out. HUGE. Yes all the students do the work together but if she is in a composite grade for eg. they may have split the kids up or they may have split them up according to ability groups etc. Also sometimes they way they ask the questions in NAPLAN is really confusing. I was working through some things on it with some teachers and there were a few questions that as teachers we had to read 2-3 times to understand so imagine how difficult it would be for a yr3 or yr5 student to figure out under stress of a "test". The language used in some of the maths questions seriously needs to be looked at.

Anyhoo tutoring may help but more so would just getting her to use maths in daily life, helping her find counting patterns, using clocks and time, helping with measurement in cooking, adding up in the supermarket, halving and dividing recipes and dinner and the list goes on. Just finding ways in everyday life to get her think mathematically will assist her more greatly than sitting down and working through a book with tutor, although if she does have very serious gaps in her knowledge then it may assist as well.

HtH.