Nilfisk Extreme domestic vacuum cleaner front castor, sold in a PACKET OF ONE, see the second picture.
A Torx 20 (T20) screwdriver is required to fit them.
This castor also fits the Nilfisk GM90 and GS90 and The GD1000 Series of commercial vacuum cleaners which includes the Nilfisk GD1005 and the Nilfisk GD1010 and the Nilfisk Business 10 litre commercial vacuum cleaner and the Nilfisk Family 5 litre barrel vacuum cleaner
They also fits the Nilfisk GM200 series, the GM300 series, the GM400 Series and the Nilfisk GM500 King series Domestic Vacuum Cleaner series as well as the Nilfisk Compact C120 C220 C330 series Vacuum Cleaner
The photo depicts the current castor, but the exploded view, shows the earlier type.
The front one is easy to replace, however, the back ones require the whole machine to be stripped, not an easy job if you've not done it before. AND ELECTRICAL COMPETENCE IS STRONGLY ADVISED
If the front wheel is good and it's the old grey type with the ball bearing in it, then it may be replaced with the new wheel, and this wheel moved to replace a single rear wheel that's broken.
In that case, carefully lever the back broken wheel, straight off, in line with the axle on which it pivots.
If you haven't busted it, you'll see how it's done. Then do the same with the front, check the C clip is still on the pivot axle and you have the ball bearing and replace these parts on the rear.
Then use a new wheel on the front of the machine.
UPDATE since mid 2014, these wheels on the 15 to 20 YEAR old machines have begun to break up, and we're asked each month to replace a couple of sets.
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!