The inter-bedded layers of sand and clayey soils of the ‘Perth Formation’ (often referred to as the ‘Guildford Formation’) underlying Perth CBD provide significant challenges for geotechnical engineers. Surprisingly, no systematic laboratory investigation of the ‘Perth Formation’ has been published to date and designers use in situ test data almost exclusively to provide parameters for foundation design. To address the shortage of element test data in the public domain, this paper presents the findings of a laboratory investigation supplemented by in situ test data for a typical 9 m thick horizon of the ‘Perth Formation’ at St Georges Terrace in the centre of Perth CBD. The paper provides results from classification tests including electron microscopy and X-ray diffractometer analyses in addition to state-of-the-art triaxial, simple shear and oedometer tests. The in situ test data are compared with these results to provide a basis for comparison at other sites in the Perth area.