Firefighters remained at the charred shell of a Virginia factory on Monday morning where a huge fire had broken out at the weekend, shrouding the city in smoke.

An emergency situation was declared on Sunday afternoon after a fire destroyed the commercial factory in Brisbane's north, sending toxic smoke billowing into the air.

Firefighters brought the fire under control after 5pm at the warehouse, just off busy Sandgate Road at Virginia, but continued to fight the blaze long after. On Monday they were still hosing down the area to prevent reignition.

A police exclusion zone was reduced about 7.30pm and smoke-affected residents advised they could finally leave their homes after close to four hours inside.