The flag hung from the cliff face above the tunnel entrance.
Police were alerted to the flag by motorists at around 6am on Thursday morning, reports news agency NTB.
“A large Nazi flag, or a flag with a swastika, was hanging over the tunnel entrance. We obviously decided to remove it, but it was hanging quite high up, so we need assistance from the fire service,” operation leader Fredrik Landsverk Mo of the South East police district told NTB.
The tunnel was closed for just under a quarter of an hour while the flag was removed with the aid of a hydraulic ladder.
Police currently have no information on who is responsible for the flag.
“Whether this has anything to do with serious [Nazi] sympathisers or is just a prank is difficult to say. We have confiscated the flag and will investigate,” Mo said to NTB.