UPDATE: An "unknown crystalline substance" that was dropped into the mailbox of a South Toowoomba home "doesn't appear to be immediately dangerous", Queensland Fire and Emergency Services Anzac Avenue Station Officer Tony Guse said.

Four crews are currently on scene at the house.

"It does appear as if we've had some sort of substance that's been dropped in a mailbox," Mr Guse said.

"It's a manufactured substance. And at this point in time we're doing some sampling. The atmosphere appears to be clear and it doesn't appear to be immediately dangerous. We'll do some further testing to determine exactly what we're going to do there."

Firefighters donned hazmat suits to assess the substance, after it was reported to emergency services by the occupants of the house around 3.30pm.

"We've got to take all the precautions and make sure our firefighters are protected and we've got to treat these substances as suspicious and noxious until we determine otherwise," Mr Guse said.

"We can do a certain amount of testing and certainly we can determine whether a substance is airborne or whether it's got any radiological or chemical biological-type principles, but further to that, we would send it away for further testing.

"Next step now is..., once we've completed the testing. We'll probably capture that product and seal it, maintain the chain of evidence through to police."

Emergency crews respond to an incident in South Toowoomba on September 17, 2018.
Emergency crews respond to an incident in South Toowoomba on September 17, 2018. Kevin Farmer

EARLIER: Emergency services have been called to a home in South Toowoomba after reports of an "unknown substance" being discovered.

A Queensland Fire and Emergency Services spokeswoman said two fire crews were on scene at Long St, after reports of an "unknown substance at that location". 

Firefighters have setup outside the home with a field sampling kit and others in hazmat suits have entered the building.

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