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As a violent earthquake shook Christchurch in the early hours of Saturday Peter Davie watched the city's lights go out.

Davie, chief executive of Lyttelton Port, was awake in bed because one of his children was feeling unwell.

"As soon as it started we said, 'Well this is serious'," he said.

"We actually live up on the hill and you could look out the bedroom window and you just saw the power going off right across Christchurch very, very quickly."

It was violent and unsettling.

"Now I understand when you look overseas and see people camping outside after an earthquake, why they do it because you're fearful that something else is going to hit."

The city was still getting reasonably sharp aftershocks yesterday morning, he said.

"It's a scary situation ... grown people are afraid and understandably."

The earthquake had caused a lot of damage at the port - which moved 9.8 million tonnes of cargo during the year ending June 30 - particularly in the area behind the wharves.

"That's the bad news that the insurance companies are going to have to pay out big time," Davie said. "The good news is that we are operating as a port."

The container terminal was operating, coal had been loaded and bulk cargo had come in.

"From an operational perspective looking forward we expect to be going at 100 per cent," he said. "It's a huge relief and I think it's a testament to the engineering work that's been done over a number of years to build the port and secondly to our guys in getting underneath and making assessments quickly and restoring power and all of those critical things for us."

The initial estimate was for ten of millions of dollars worth of damage, although the insurance assessors were expected to start work yesterday.

Earthquake protection is something taken into account when developing the port.

"We build all of our structures around earthquakes and when you think of container cranes which are 50 [or] 60 metres in the air they can still withstand those type of shocks is a testament to good engineering work."

Most of the wharves have a system of large cables running back to huge blocks of concrete buried in the soil called 'dead men'.

The concrete blocks had ripped violently back and forward destabilising the paving.

"It's done exactly what our engineers predicted would happen in a big earthquake so that's reassuring in itself," Davie said.

Road links into the port were fine, the main south rail line was working and the west coast line was expected to be operable from yesterday evening.

"It looks like we can move goods in and out," Davie said. "We've really got to think now about what areas can and can't receive those goods."

Areas of the city closed did not tend to be the industrial parts, he said.

"We think that overall we don't expect to see a huge disruption."

Dairy exporter Fonterra said it had been able to collect all milk.

There were no issues at the company's Takaka, Brightwater, Kaikoura, Clandeboye and Stirling sites. The Plains site suffered some minor damage but production re-started on Sunday.

"With Port of Lyttelton experiencing minimal damage we don't expect any issues with dispatching orders to our overseas customers," Fonterra said.

Pike River Coal said its second export shipment of about 20,000 tonnes of premium hard coking coal worth about $6 million departed Lyttelton Port for India yesterday, one day later than planned.