If driving with a bike rack on your vehicle that obscures your number plate, an auxiliary plate is needed.  

It is commonly thought that displaying a handwritten number plate is enough, however this is illegal. A clearly displayed handwritten number plate can still incur a fine and a loss of demerit points.  Number plates and tail lights need to be clearly visible. Therefore, a light board may be needed as well.

Please also note that is illegal for a bike rack to stay on a vehicle if not in use.