Ottawa Mayor Jim Watson says it could take up to two months to determine what caused a giant sinkhole to open up blocks from Parliament Hill, swallowing sidewalks and a car as it grew.

The massive cavity opened up on Rideau Street near Sussex Drive on Wednesday morning, causing a gas leak and forcing a number of nearby buildings to evacuate.

"You can be looking at two months to find the final cause of the sinkhole," Watson said during a news conference on Thursday.

Tunnelling work for the Ottawa LRT was taking place about 20 metres below Rideau Street when the sinkhole formed. Construction crews were aware of the soft soil conditions in the area and had taken “extensive precautions” to stabilize the ground.

Crews worked throughout Wednesday night and into Thursday morning to fill the hole, which is estimated to be 28 by 40 metres and about five metres deep.

Watson said 2,700 cubic metres of concrete – the equivalent of 300 cement trucks – has been poured into the sinkhole to stabilize the area.

Footage of Rideau Street collapsing into the ground, swallowing up sidewalks and a van, was widely shared on Wednesday after the sinkhole opened up.

City officials said Thursday that crews were unable to remove the van from the sinkhole before filling it with concrete, as the surrounding ground was too unstable to support the equipment needed.

Nearby buildings, including the Rideau Centre mall and the Shaw Centre, were evacuated following a gas leak. 

Rideau Street is not expected to be fully re-opened for another week or two, but Watson said the repairs should be completed in time for Canada Day celebrations in the city, when the street serves as a busy pedestrian corridor.

Ottawa residents, meanwhile, have been told to prepare themselves for "significant" traffic delays as a number of downtown streets remain off limits to traffic and road repairs get underway.

Officials said a precautionary boil water advisory is in effect for some business in the downtown core.

The city also warned residents who live in the downtown core that their water may be discoloured and the taste could be different, but it's still considered safe to drink.