Police from Lower Austria say the cannabis farm was run by a 38-year-old man, believed to be a postman.
He was arrested on Wednesday, as he was handing over some cannabis plants to a younger woman.
Police think the cannabis farm was being run as a professional business, according to the Austrian Press Agency. The sniffer dog, called Frankie, found containers filled with harvested cannabis. Police did not disclose how much undelivered post was found in the flat.
The 38-year-old has been taken to Vienna's Josefstadt prison, and the woman received a warning.
Production and possession of cannabis is illegal in Austria - with prison sentences of up to one year for possession, although offenders can complete probation or drug rehabilitation instead.
However producing large amounts, over 200 grams, is considered a serious crime and can carry a sentence of up to ten years.
Final score: dog 1, postman 0.