andysh: I think you get taxed on the tax rebate, good eh
In general, with a simple tax situation like yours, I would expect the IRD to get things right, but let's take a look (n.b. I am by no means a tax expert, but I don't really think you need one here).
Assuming you were taxed (correctly) at 19.5c and 1.3c ACC earner's levy then your taxable income should have been around $5,600, right? You would have been eligible for the child rebate (if you were at school all year) and the under $38,000 rebate. The under $38,000 rebate brings your tax rate down to 15% for the first $9,500 (i.e. rebate of ~$250) and the child rebate means the first $2,340 was tax free (i.e. rebate of $351, the maximum). So, seems like the $600 is about right.
Were you hoping to pay no tax?!
P.S. Any tax experts feel free to chime in if my calculations or assumptions are incorrect.