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A new, ultra long-spanning, combined steel formwork and reinforcement system has been developed in conjunction with the University of Western Sydney (Australia) and is designed to span in excess of eight metres without propping. The hybrid steel deck comprises a cellular module incorporating a top plate with multiple longitudinal stiffeners that is mechanically connected to a base panel through vertically-corrugated webs. All of these elements can have different thicknesses and steel grades. To establish the mechanical connections between the components of the main decking panel, round press-joints (TOX®) are used. The webs can have pre-punched holes to partially or completely fill the closed cellular section with concrete and to allow reinforcing bars and/or prestressing cables be passed transversely. The modules, which can vary in their overall height, can be precambered to control deflection under the weight of the wet concrete. Small-scale tests have been carried out to understand the basic behaviour (shear and combined shear-tension) of these unusual mechanical connectors, as well as their effect on steel performance, in particular on high-strength steel (G550). In a series of preliminary positive bending tests, a range of principal failure modes could be identified, of which the longitudinal shear failure of the TOX® connectors in the base panel (BPSH) and the buckling of the top plate under compression (TPBU) were most common. These two failure modes were further investigated in an extensive series of flexural tests in which the material combinations and the spacing of the connectors was varied. For the BPSH failure, measurements of longitudinal slip showed a fundamentally different distribution compared to the conventional theory used for composite steel-concrete beams. A simple computational model was developed to predict the deformation performance, whereas the ultimate moment capacity could be well predicted by using a partial shear connection model in conjunction with elastic theory. For the TPBU failure, digital close-range photogrammetry was employed to measure the top plate deformations. Notwithstanding the different geometries and material grades used for the top plates, increasing the connector spacing did not significantly reduce the plate strength. Specimens with a low strength top plate material were able to reach compressive stresses close to yield of the material. Simple design models, based on the results of the finite strip method and an elastic member buckling analysis, have been established to predict the observed buckling behaviour of the top plate. The predicted behaviours were found to be sensitive to the modelling of the hybrid steel deck.