Much of Christchurch is without essential services after yesterday's earthquake with gas, power, water and phone infrastructure all affected by the 6.3-magnitude tremors.

Contact Energy turned off the gas supply to the city last night and was advising residents to turn off their mains connection.

The company will restore supply only when it is safe to do so, said spokeswoman Janet Carson.

Civil Defence reported freshwater and sewage pipes had cracked and water was pouring into the streets.

It is not known how many households are not connected to the water supply.

Both cellphone and fixed line telephone services struggled to shoulder the extra load yesterday as families and friends tried to contact one another after the earthquake.

The 111 emergency line was down immediately after the quake but service was restored when calls were diverted to a Wellington centre.

Power outages meant many landline calls in Christchurch could not be connected, putting extra strain on mobile networks.

Overloading and network congestion caused calls and texts around the country to fail, with local calls as far afield as Auckland reportedly cutting out. Eftpos services around the country were affected by the line congestion and clients were told to expect delays.

Telecom's head of external communications, Mark Watts, said the fixed line network was "holding up pretty well" but the mobile network could deteriorate overnight as cellphone towers around the region were running on back-up batteries after power failures.

The situation was much the same with Vodafone. Communications manager Matt East said eight sites out of 150 were down and 43 were disconnected from the power supply and running on back-up batteries.

"If we don't get the power back on they will go down and there might be patches [of Christchurch] without coverage," he said.

Telcos were advising the whole country to keep phone traffic to a minimum so emergency calls in Christchurch could connect.

When mobile networks overloaded, concerned family and friends used micro-blogging site Twitter to try to contact one another and gather information about their loved ones. The Twitter hashtag #eqnz, which users put on the end of their messages, meant news on the disaster was easily searchable.

Christchurch residents tweeted reports and photos of the devastated buildings after the quake struck.

Google set up a people-finding service, where internet users could register if they were safe and request information on friends.