This paper investigates the reliability of the existing design equations specified in AS/NZS 4600:1996 for fillet welded connections in cold-reduced high-strength G450 sheet steels. The existing design equations are adapted from the AWS D1.3 Structural Welding Code, which is based on the testing results of double-lap connections in mainly mild sheet steels. Double-lap and single lap fillet welded connections in 1.5-mm and 3.0-mm sheet steels are manufactured using different GMAW procedures and tested to failure. The approximate tensile strengths of the heat-affected-zones (HAZs), which are significantly lower than the corresponding tensile strengths of the virgin sheet steels, are used to predict the failure loads of the specimens. The HAZ strengths are close to the nominal design tensile strength of 480 MPa specified in AS/NZS 4600:1996. Despite this fortuitous agreement, the lack of ductility of G450 sheet steels compared to mild steels leads to earlier failure of the fillet welded connections loaded in the longitudinal direction of the welds. Failure modes of different types of connections and of connections in different sheet thicknesses are described and discussed. Relaxation of the target safety index for longitudinal fillet welded connections is proposed. It is suggested that the existing design rules may be applied conservatively to transverse fillet welded connections which do not undergo inclination failure, and to longitudinal fillet welded connections.