Many countries charge a fee to leave depending on whether you’re exiting by air, land or sea. These charges can vary on where you’re from and not all airlines/countries add them automatically to your tickets, so you’ll need to have enough currency with you at your departure point to pay. As examples, Oz add it to air tickets as standard; NZ don’t but have handy ATM-style machines to make payment on plastic very simple; Vietnam requires payment by cash in Dong or US Dollars and their charge varies dependant on your nationality.