Hi David - I am certainly not aware of this. Only Residents and Tunisian passport holders must pay a 'departure' tax and that is TND60.000. I am flying next week and will check with the douane at the airport.
An update on departure tax for tourists/foreigners..............I have asked the douane (customs) at the airport in Tunis if there is a new departure tax for the above. The answer was NO!! So you are all lucky you don't need to pay anything on departure. You may be confused with a stamp of TND60.000 that citizens and residents need to pay on departure.
Tunisian or not a Tunisian (National) it does not matter, if you are a Tunisian resident or you have reside more than 6 months you have to pay a tax departure upon departure from the airport, if you have been into Tunisia for a less than 6 months you are exempt from paying this tax! It is a tax on resident of Tunisia, that does not concern visitors who are not staying more than 6 months (for that purpose if you are not a Tunisian National you have to have a residency permit to be applied for from a police precinct, and the latest fee of the tax departure is 60 dinars! = $45 US)