A weld-on hinge (sometimes 'weld hinge' or 'bullet hinge') has two cylindrical halves: a male half with the pin facing up, a female half that drops over the pin. Each half has a steel tab or saddle that gets welded straight to the steel work. No plates, no bolts.
Weld-on hinges shine on heavy steel gates because every load goes through a continuous weld instead of through a bolted plate. Common Australian sizes are 16, 20, and 25 mm pin diameter. A 20 mm weld-on hinge in pairs handles a single-leaf swing gate up to about 200 kg. For 350 kg-plus leaves, three hinges per leaf with a 25 mm pin is the usual call.
Weld-ons need careful alignment during fit-up because adjustment after welding is limited to grinding off and starting again. Tack-weld first, hang the gate, then full-weld once you've confirmed plumb.