It would depend on what purpose you are seeking a GPA/WAM.
Generally when you transfer, subjects you have been granted credit for from a previous uni will appear on your new transcript, but won't have a mark. And if you are calculating marks for honours etc, then generally uni's will only count subjects that you have undertaken with them.
However if you apply for a job and the employer wants your WAM, and the subjects you did at the previous university are relevant to the new degree, then I would think you would want to calculate your WAM/GPA including those marks from the previous university (especially if they ask for transcripts etc).
I know at my uni (USYD) they don't calculate your WAM or GPA for you. No WAM or GPA appears on your transcript. If an employer asks for your WAM, you calculate it yourself. In that regard, I would think you would include all subjects that have contributed towards that degree (or the field the employer is interested in), whether or not the current uni counts it in their GPA/WAM.
If you transfer and you don't have any subjects counting for credit, then its unlikely an employer would care about it, in which case don't include them in your GPA/WAM.