Unfortunately, from time-to-time concrete can be struck by hairline cracking or cracks. Hairline or cracking concrete is common and generally not a cause for concern. Common causes of cracking include shrinkage stress, stress due to trees, commercial or heavy vehicles, soil movement due to changes in moisture content due to garden watering or drainage problem. The QBCC Guidline has put together what is tolerable.
For further reference to QBCC Guidlines, please visit: www.qbcc.qld.gov.au/sites/default/files/Standards_and_Tolerances_Guide