SMALL amounts of a non-toxic gas will be pumped through Melbourne's underground railway stations next week, as part of preparations in case of a major emergency.

On Wednesday and Thursday next week, the Department of Transport will work with the fire brigade and other emergency agencies to release sulphur hexafluoride into the three City Loop stations - Parliament, Melbourne Central and Flagstaff.

An odourless gas will be released into three City Loop stations, including Flagstaff, next week. Credit:Craig Abraham

Releasing the odourless gas into the air flows will allow the fire brigade, the federal government, and local rail authorities to know how oxygen and gases disperse in the tunnels in the event of a fire, gas leak or terrorist attack.

To model what the gas does when it is released, staff with ambient air analysers will take samples around the stations, to register where the gas disperses to and in what quantities.