WELLINGTON, Nov. 16 (Xinhua) -- The first shipload of refugees left the quake-stricken town of Kaikoura Wednesday as the New Zealand government announced the magnitude of Monday's deadly tremor had been upgraded from 7.5 to 7.8.

The government's GNS Science institute had reassessed data of the quake that struck just after midnight Monday from its monitoring stations across the country, Acting Civil Defence Minister Gerry Brownlee said in a statement.

"Because it took over a minute for the fault to rupture during this event, the standard method normally used to calculate the energy released during an earthquake was insufficient," said Brownlee.

"As expected with a larger earthquake, the revised magnitude does have an effect on the probabilities of forecast aftershocks, meaning it is now forecast that aftershocks may be larger in magnitude for a longer period of time."

Meanwhile, New Zealand navy supply ship HMNZS Canterbury was due to set sail late Wednesday for the port of Lyttelton, near Christchurch, with about 390 evacuees from Kaikoura on board.

"The ship is expected to arrive around 11pm tonight, depending on weather conditions and the amount of time it takes to upload those waiting in Kaikoura," said Brownlee.

"The New Zealand Defence Force (NZDF) has now evacuated about 600 people from Kaikoura. Evacuations will continue for as long as necessary and, as of tomorrow, several ships from other nations - including the (U.S. Navy destroyer) USS Sampson - will arrive off the Kaikoura coast ready to be mobilized as required."

The southern road route into Kaikoura, which was cut off by massive landslides after the quake, was expected to be cleared by the weekend, but police would control access was it reopened.

Air Commodore Darryn Webb, the Acting Commander Joint Forces New Zealand, said other navy vessels helping in the relief effort included offshore patrol vessel HMNZS Wellington, frigate HMNZS Te Kaha and replenishment tanker HMNZS Endeavour.

"We have been working with the Ministry of Civil Defence in ensuring emergency needs in the affected communities are met promptly, and we are ready to provide additional support as required," Webb said in a statement.

A New Zealand air force C-130 Hercules aircraft transported a generator, a water purification system and about 5,000 liters of water to Kaikoura Wednesday.

Four air force NH90 helicopters rescued 200 people on Tuesday and have been delivering emergency supplies.

The NZDF forces at Kaikoura would be joined over the next day by Australian, Canadian and U.S. warships with their embarked helicopters.

The quake, which killed two people, trapped about 1,000 tourists in Kaikoura, which is famed for its coastal scenery and whale-watching activities.

Foreign vessels in the relief operation had been redeployed from navy detachments from at least 15 countries, which had been scheduled to attend an international naval review to mark the New Zealand navy's 75th anniversary in Auckland from Nov. 17 to 22.