Trams and cars were diverted from the intersection of Spencer and Collins streets for several hours during the day while the leak was contained.

Emergency crews have since left the scene and Collins street was re-opened in both directions about 2pm.

APA Group, which manages the gas network, said the leak had been contained just before 1pm, but that it was not yet known what caused the leak or when repairs would be complete.

Air-monitoring equipment earlier showed the gas, which was pouring out of a ruptured main underneath Collins Street, was reaching "lower explosive limits".

"it's quite high. It's close to lower explosive limit," Metropolitan Fire Brigade incident controller Dick Grant said.

"But you've got to get right up close to the holes in the covers. But a metre away, it's almost insignificant because of the wind surge.

"We're not too concerned about the immediate area. We're not getting high readings away from the leak but over close to it we're getting some high readings, so as a precaution we have sealed the area off."

The gas leak is coming from under these metal plates. Photo: Liam Mannix

The gas leak was first noticed about 6am.

It did not affect train services at Southern Cross Station.

The street closure affected business at the nearby 7-Eleven store.

"We haven't had to close, but no one is coming in because the street is closed," 7-Eleven worker Mourad Hanania said about 10am.

"I may not have customers for another two hours."

Mr Hanania said people outside his store could smell gas.

Equipment arrives to dig up Collins Street. Photo: Liam Mannix

Collins Street cafe Purple Peanuts was forced to close its doors about 10am as firefighters tried to stem the leak.

"I could smell the gas outside," one worker said.

"We can stay inside the shop, but we've had to close the door. We can't serve customers at the moment."

Catherine, who did not want her surname used, was working the counter at Perkup Espresso, which was empty.

"I really hope it doesn't explode", she said, holding up crossed fingers.

At Krispy Kreme, on the opposite side of the street, it was business as usual but staff noticed a few extra customers coming from stranded trams.

With Robyn Grace, Marissa Calligeros, Allison Worrall