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
As it's light, maximise the saving on postage and buy some of the related parts below, such as dustbags and the filters or a new floor tool.
At the moment, standard postage within Australia is capped at a maximum of $20!