G450 steel to AS 1397 is a cold-reduced sheet steel with in-line galvanizing. Its grade is 65 ksi (450 MPa) yield and 70 ksi (480 MPa) tensile strength. It is widely used in Australia for purlins, and is being used to fabricate light-weight portal frames, often by welding. The effect of welding on G450 sheet steel in the heat affected zone was unknown and so the project was performed to investigate the strength of fillet welded connections. Transverse and longitudinal fillet welded connections in 0.06 in (1.5 mm) and 0.12 in (3.0 mm) sheet steels were tested to failure. The failure modes and ductility of different types of connections are described. The test results are used to check the design rules in the AISI Specification (Section E2.4) and the Australian/New Zealand Standard for Cold-Formed Steel Structures AS/NZS 4600 (Clause 5.2.3). Recommendations are made for revised capacity factors. The quality of fillet welded connections in thin sheet steels produced by industry fabricators is investigated. The need to complement the macro test with the destructive prying test as part of the pre-qualification procedure for such connections is demonstrated.