Aggregates used in pavement construction in New Zealand are classified as either premium or marginal depending on their quality. Using marginal materials in a pavement may cause significant problems, such as fatigue cracking, rutting and swelling. One likely reason for these problems can be the mineralogical composition of the aggregates, particularly if they contain swelling minerals. The present paper investigates how swelling minerals can affect the properties of New Zealand aggregates. Standard engineering tests were conducted on five New Zealand materials, two of which are regarded as meeting the local NZTA (New Zealand Transport Agency) M4 premium specification and three regarded as marginal. The geological properties and mineralogical compositions of all aggregates were also determined by petrographic examination and X-ray powder diffraction analysis (XRD), and the swelling potential identified using a one-dimensional swelling test. It was found that clay minerals generally exist in all five aggregates and swelling minerals in the three marginal aggregates and a premium aggregate; the last mentioned also proved to have a high swelling potential. It is concluded that the test methods specified by the NZ standard and currently used to distinguish between and categorize premium and marginal aggregates appear insufficient when they are water saturated. Additional test methods (XRD and a swelling test) are suggested to complement the standard test methods.