Here is the link to the NAPLAN website:
http://www.vcaa.vic.edu.au/prep10/naplan/index.html

And here is the link to the sample Numeracy questions:
http://www.naplan.edu.au/test_samples/numeracy/numeracy.html

And the link to AIM sample questions from Victoria (which presumably are good practice for the NNA):
http://www.vcaa.vic.edu.au/prep10/aim/testing/index.html