CFA operations officer Paul Carrigg said: "The elderly occupant was a retired chemist who had lived there for over a decade and who appears to have been collecting chemicals since his school days so it was a 60-odd year collection.

"I haven't seen anything of this extent before. To find so many chemicals in residential premises and of such high-risk types, it's a strange event.

"Some containers were leaking and a lot were unstable because of their age. It absolutely put other residents in the block of flats at risk.

"I think it was only sheer luck there wasn't a mishap and a consequent explosion."

The potentially explosive situation was uncovered by cleaners who had been asked by a real estate agent to clean up the Cranbourne Road premises, which were recently vacated by the retired chemist.

Three shelves crammed full of chemicals were found in the kitchen when firefighters arrived about noon on Tuesday last week.

Even more dangerous, the cleaners unwittingly moved a refrigerator chock-full of chemicals outside without realising the danger.

Mr Carrigg said the painstaking, 10-hour clean-up required about 40 CFA volunteers, many dressed in protective clothing, and technology including air quality monitors.

They had to sort and classify the chemicals and pack them into hazardous material drums, which were removed by a transport company for safe disposal.

The occupants of the two-storey block were not allowed back until the all-clear was given. Brigades from Dandenong and Hallam also attended.