A building system (5) including a panel system (10) and a formwork system (78). The panel system (10) includes a plurality of modular panels (12) arranged to fit with one another in a predetermined grid arrangement (13) to form side walls of a building in a fitted condition. At least the bottom sides (24) of each of the modular panels (12) includes a plurality of apertures (34) that are arranged at predetermined spaced apart locations along the bottom side (24) so as to coincide with the predetermined grid arrangement (13). The formwork system (78) includes a plurality of modular formwork members (80) that are associated with each of the modular panels (12) to provide a supporting slab (72) having slab apertures (76) to allow coupling with the modular panels (12). Modular panels (12), formwork members (80) and associated methods of use are also disclosed.