A sat nav will give km/h however; unless it is a specific unit for some predesigned purpose, like a Vapour dash for a motorbike, I doubt it will record your total kms travelled.
And yes, your vehicle must have a working odometer and if it is replaced with something else you will need permission for that. Talk to the Consumer and Business Services in your state, they will have a form to fill out if you need to alter the odometer reading for any reason.
