If it isn't stamped then you don't have a real lease or any rights under the law.
Sounds like your landlord is trying to get out of paying taxes. Once it's stamped its on record with IRD and my thoughts are that they will check every single one to make sure they are getting their 15%.
You can tell the agent you will go to EAA to ask why the agent isn't helping. Maybe that will get his act together but the landlord should know its stamped. You never know the agent might have got you a lower price for it not being stamped. Yes it's wrong but I would communicate with everyone that the law requires it stamped so will be doing that.