The first event, a 7.8-magnitude quake, struck just after midnight Monday near the coastal community of Kaikoura, some 93 kilometers (55 miles) northeast of the city of Christchurch, the US Geological Survey reported.
It triggered waves of 2.49 meters (8 feet) above usual tide levels, the highest he has seen in his 38 years in New Zealand, Philip Duncan of Weather Watch New Zealand said.
An initial tsunami warning was lifted but temblors continued Monday afternoon. A 6.2-magnitude quake struck around 1:30 p.m. 39 kilometers west-southwest of Kaikoura, further north of Christchurch.
Prime Minister John Key posted a short video to YouTube after surveying the damage.
"It's hard to see how those roads, neither north nor south, will be opened very easily," he said.
"In the short term what we're trying to do is make sure that the people of Kaikoura and the likes have food, water, help and backup.
"In the short term what we're trying to do is make sure that the people of Kaikoura and the likes have food, water, help and backup.
"I'd say it's billions of dollars worth of money that we have to spend here but we're focused on what we're doing and we'll keep up the good work."
New Zealand is a South Pacific nation of islands, the two largest being the North and South islands, which are home to most of the population. Aftershocks from South Island reverberated all the way to Wellington, the country's capital on the North Island, where residents were told to stay indoors Monday.
Watch a video below of a house on the fault line that was forced in two:
