In addition, any excess embies produced by donor sperm, cannot be on-donated (which is the thing I found extremely difficult).
I asked this question of my clinic counsellor a few years ago and she said that the recipients could donate the embryo and that I had no input into the matter. The SD looses all rights to the sperm once it is utilised to fertilise an egg. So the recipient couple could then donate the embryo to another couple without the need for the consent of the SD.

This is a possible "grey area" for the new NSW legislation coming out ...

My personal opinion is that I am happy for the recipient couple to donate the embryo to another couple, but it is a little weird that I do not get a say in the matter. I very much doubt the clinic would even inform me of the situation.