Gala Equine follows Australian rug sizing:

Australian/UK sizing measures from the middle of the chest, along the horse's side and back to where the rug should end.

To measure your horse for a NZ rug, you need to measure from the horse's wither to tail in a straight line. 

Size difference tip: Australian/UK rugs are typically 1’3” longer than the equivalent NZ measurement. For instance, a NZ 5’6” rug would generally match an Australian/UK 6’9”.

Please note, this chart is intended as a guide only — every horse is built differently, with variations in height, width, and wither shape.