There are two answers to this question; one being the legal answer and the other being what can happen in reality. It is not too different from court name suppression issues that sometimes arise in the media.
A Court of Appeal decision reconfirmed that, the happenings at a mediation are considered the same as "Without Prejudice" negotiations when resolving a dispute. In other words, any act, admission and/or statement made during mediation is inadmissible as evidence in a later judicial process. Opponents to this started publishing editorials making comments like,
"Say whatever you like about whoever you like, so long as it is in mediation!"
One of the reasons mediation is so effective in resolving disputes is that the parties can talk openly and freely, without the fear of this free speech coming back to haunt them at a later date. It allows a mediator to explore, in depth, the real and underlying issues at hand in order to help the parties come up with their own creative solution.
So, yes, you can be assured that what happens in mediation cannot be used as evidence against you at a later date. Don't forget that this works both ways. You can't use information gained at mediation against your opponent either.
How, then, do I stop the other party opening their big mouth and telling others about what was discussed in the mediation? In employment matters, if you have evidence that will stand up to the tests of our courts, then you can take action through the Employment Relations Authority. Should they rule that a party has broken confidentiality, then they can penalise the offending party and give part of the penalty to the aggrieved party.