Garbage, empty food tins and stuffy rooms -- the Munich apartment where some 1,500 modernist masterpieces stolen by the Nazis were found was a "catastrophe" for storing art, a witness told AFP on Monday.

The person, who spoke on condition of anonymity, said she was with customs authorities in early 2011 when they searched the flat of Cornelius Gurlitt, the 80-year-old son of a prominent Nazi-era art dealer.

She said the modest home in the southern German city was totally unsuited for works by the likes of Picasso, Matisse and Chagall estimated to be worth one billion euros ($1.3 billion dollars).

"The climate conditions were a catastrophe for storing art," she said.

The paintings were exposed to cool air that was too dry, but although some of the frames looked chipped, fortunately "the works were not badly damaged", she added.

"It was clear he was a pack rat -- there were 50 bags from Ludwig Beck (a department store). The blinds were pulled down and the only window bringing in light was in the kitchen," she said.

Trash and discarded food packaging lay around the apartment "but not up to the ceiling", as had been reported by German media.

"It was an apartment furnished normally with a kitchen, a sitting room, a bedroom and a third room," she said.

It was in the third room that the paintings lay out of sight for prying eyes, on hand-built shelves behind a curtain.

Although the raid took place two and a half years ago, the case came to light only on Sunday with a report in the news weekly Focus.

The magazine said the works were now stored safely in a customs warehouse outside Munich as an investigation continues.

Art collector Hildebrand Gurlitt acquired the paintings looted by the Nazis during the 1930s and 1940s.

Despite having a Jewish grandmother, he had become indispensable to officials in the Third Reich because of his art expertise and vast network of contacts and was tasked with selling the works abroad.

However Gurlitt apparently secretly hoarded many of the works and claimed after the war that the masterpieces were destroyed during a wartime bombing raid on his Dresden flat.

After his death, his son, a recluse without a job, held onto the collection, allegedly selling a few paintings over the years and living off the proceeds.