With the nzbget that came installed with the NMT apps it did not unrar for me unfortunately. So I went looking for a solution and found that many people recommended installing Oversight using NMT CSI (Community Service Installer) because it had a better postprocessing / unpacking script.
After installing Oversight (and rebooting), the nzbget web interface was not reachable on port 8066 anymore however. After deinstallation of Oversight in CSI it started working again.
To get it all working I installed Oversight again using CSI and then (again using CSI) I stopped Oversight and configured it not to start on boot. Then everything worked as desired. Note that your completed downloads will be moved from the Downloads folder to the Completed