The first reports of serious injuries following the massive earthquake that struck Christchurch have started to emerge.

Two men are reported to be in a serious condition in Christchurch Hospital, one was hit by a falling chimney and the other was hurt by breaking glass.

The extent of the damage caused by a massive 7.4 magnitude earthquake that struck 30km west of Christchurch this morning is horrendous.

According to locals the city looks like a warzone with extensive damage to buildings and houses and most residents reporting some damage.

Walls and roofs have collapsed, some buildings in the city centre have been completely destroyed and most people are without power.

Shop and house windows have been blown in and the central city has been shut down because of safety concerns and looters taking advantage of the destruction.

Water supplies are also understood to have been cut to parts of the city and wider area.

Roads in the Canterbury region are understood to have been affected by serious damage.

Christchurch International Airport has been closed in order to check the runway for any damage.

The Canterbury District Health Board has asked people to avoid going to Christchurch Hospital unless they are suffering from serious injuries and to go to medical centres instead.

The Pacific Tsunami Warning Centre said no destructive widespread tsunami threat existed based on historical earthquake and tsunami data but earthquakes of this size sometimes generated local tsunamis that could be destructive along coasts located within 100km of the earthquake epicentre.

The quake struck at 4.35am and was initially estimated to be at a depth of 33km but that has been recalculated to be only 10km deep.

Civil defence said in a statement shortly after the quake struck that people in the affected area should:

1. Expect aftershocks. Each time one is felt, drop, cover, and hold on.
2. Check yourself first for injuries and get first aid if necessary before helping injured or trapped persons.
3. Assess your home or workplace for damage. If the building appears unsafe get everyone out. Use the stairs, not an elevator and when outside, watch out for fallen power lines or broken gas lines. Stay out of damaged areas.
4. Look for and extinguish small fires if safe to do so. Fire is a significant hazard following earthquakes.
5. Listen to the radio for updated emergency information and instructions.
6. Do not overload the phone lines with non-emergency calls.
7. Help people who require special assistance - infants, elderly people, those without transportation, large families who may need additional help, people with disabilities, and the people who care for them.