Plumbers and security staff had been searching for several hours for the source of a water leak which had spread to businesses and a café in the building in the Grønland area of the city.
With the help of a locksmith they were able to enter the apartment only to find what was described as a "jungle" of cannabis plants, according to a report in the Dagbladet daily.
"The strong stench that hit us made us first think that there was a dead body in there," said one of the group to the newspaper.
But on closer inspection the pungent aroma was found to be coming from a plantation of cannabis complete with heat lamps, watering system and air conditioning equipment. Bags of soil and fertilizer were also found on the floor.
"It looked like a jungle in there. I thought I was in a movie set," a witness said.
It has been concluded that that the advanced watering system in the apartment was the cause of the extensive water leak in the building.
Two people were later arrested in connection with the find and police expect to make further arrests following an investigation.