A suspicious fire at a waste and recycling plant at Wingfield in Adelaide's north-west has left a damage bill of $6 million.

A large shed, three excavators and more than 300 tonnes of cardboard and rubbish were damaged during by the fire, which started about 2:00am.

The Metropolitan Fire Service said the fire was suspicious. They said smoke coming from the blaze could be toxic and people living in the area should stay indoors.

Dozens of firefighters were at the scene for several hours, while the EPA was also called in to help minimise the impact on local waterways.

Assistant chief fire officer Phil Kilsby said crews would continue to monitor the fire throughout the rest of the afternoon.

"The MFS will be here for a little while at the moment. We're taking the remaining sheets off the structure to try and gain access to the fire scene and to allow for ventilation," he said.

There was another fire on Friday evening at a Salvation Army store at Noarlunga, which caused about $1.5 million damage.

The fire caused extensive damage to the back of the business, with surrounding businesses affected by smoke and water from a burst pipe.

It is believed the fire was sparked by an electrical fault in a fan.