Christchurch City Council says the main burst about 6.30am today.
It has left a hole in the road surface on Fendalton Rd.
A section of the busy arterial route is closed while contractors try to fix the problem.
Water can be seen flowing into nearby streets and workers are trying to pump it out of the hole.
The council expects the repair work to take until about 2pm.
Earlier, traffic was backed up around Fendalton Rd.
Police say they received reports of water, flooding and potential road damage on Friday at about 6.20am.
Nearby resident Rachel Alexander says she woke up to a text from a neighbour warning that the water was off.
While walking her dog she says she saw the sinkhole in the road.
"I managed to get right up next to it and the sinkhole is a good one metre or two metres wide and there's a contractor digging there."
The burst water main is near St Barnabas Church, a prominent landmark in the city.
The city was hit by heavy rain on Thursday.
