Almost all of the island's 3 million households were left without power.
The quake left nearly 3 million people without power after damage to a major thermal plant supplying the region, with Japanese Minister of Economy, Trade and Industry Hiroshige Seko saying it could take "at least a week" for supply to be restored.
Some 1.6 million residents across Hokkaido remain without power. In Atsuma, a town of about 4,600 people, 26 were still unaccounted for. Eight people are reported to have died, according to local media, while there are around 40 now missing.
"Please give your sympathy to people who spent a dark night in fear, and do everything you can to restore electricity as soon as possible", Prime Minister Shinzo Abe told a cabinet meeting to discuss the quake.
"We're trying to do it faster, but it will likely take a week", Seko said. Local buses and trains and bullet train services were halted. "All the traffic lights are out and there's no power at work".
Abe also warned residents to "exercise extreme caution" amid forecasts for more rain, which could cause further landslides, while officials warned of the dangers of more quakes.
Moreover, Hokkaido's main airport, New Chitose Airport, was shut in the aftermath of the quake, which struck nearby. Debris and water could be seen on the terminal floors. The government will send 20,000 more to the affected sites, he said.
Japan is still recovering from the worst typhoon to hit the country in 25 years, which struck the western part of the country on Tuesday, claiming at least 11 lives and causing major damage to the region's main airport.
Flights resumed from midday at Hokkaido's main airport, New Chitose, with more flights planned for the afternoon, airline officials said.
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Farming, tourism and other services are big economic drivers on Hokkaido, which accounts for just 3.6 percent of Japan's GDP, but there is some industry.
While not as many people were affected by this week's outage as after the March 2011 quake and tsunami, it was the first time a regional utility in Japan lost its full network, according to the country's Federation of Electric Power Companies.
Hokkaido's Tomari nuclear power station, which has been shut since the 2011 quake and tsunami, suffered a power outage but officials said it was cooling its spent nuclear fuel safely.
A fire that broke out at a Mitsubishi Steel Mfg Co plant in the city of Muroran after the quake was extinguished with no injuries.
An aftershock measuring 5.3 rocked the area moments later and smaller aftershocks followed throughout the night.
Japan sits on the Pacific "Ring of Fire" where numerous world's earthquakes and volcanic eruptions are recorded.
The powerful natural disaster and tsunami in March 2011 that hit northerneast Japan destroyed both external and backup power to the Fukushima Dai-ichi nuclear plant, causing meltdowns.
The tsunami also damaged the Fukushima Daiichi nuclear power plant, leading to a series of explosions and meltdowns that spewed radiation into the air and ocean. Seismologists have said another such quake could strike the city at any time.