Nilfisk GD5 Commercial backpack vacuum cleaner shoulder harness strap kit, COMPLETE it will also fit the Nilfisk GD10.
(Yes, there is a premium kit 147-1246-510 $210 but expect a months wait)
Please Note: The "A" frame back support alone is PN. 147-1091-500
Maximise the saving on postage and buy several Puregiene toilet rolls a box of Tissues and some soap or maybe some packets of dustbags, or a hose or a new floor tool, or some other accessories!
At the moment, standard postage within Australia is capped at a maximum of $20!