Southwest Airlines has new fuel-efficient planes coming that would allow the carrier to add more destinations to its lineup of flights between Hawaii and the mainland, a spokesman said Wednesday.

The low-fare airline’s launch of service to Hawaii this week has been wildly successful, which could lead to more investments.

“Some flights were sold out in minutes. It took three or four hours to sell out of weeks of flying,” said Southwest Airlines spokesman Brad Hawkins in an interview with the Honolulu Star-Advertiser.

To fill planes quickly, Southwest kicked off its Hawaii service with introductory low fares offering transpacific flights for as little as $49 each way and interisland sales from $29.

Southwest’s current ramp up into Hawaii and future expansion will be based on consumer demand, said Steven Swan, Southwest’s senior director strategic planning and airline partnerships, in the interview.

“In theory, we could have more Hawaii announcements this summer or later this year. It will depend if it’s an investment that our leadership team feels is worth making. We’re not far enough into the Hawaii story to know where we would end up,” Hawkins said.

Gary Kelly, chairman and CEO of Southwest Airlines, already has confirmed Las Vegas and Phoenix were on a list of future Hawaii opportunities.

Southwest will make its Hawaii debut March 17 with a sold-out flight between Oakland International Airport and Daniel K. Inouye International Airport. It will start service between Oakland and Maui on April 7. The carrier will begin flying between San Jose, Calif., and Honolulu on May 5, with service between San Jose and Maui starting May 26.

On April 28, Southwest will launch interisland service, four times daily in each direction, between Honolulu and Kahului. Beginning May 12, the carrier will begin service between Honolulu and Kona flying planes four times daily in each direction.

By the end of May, Southwest already will have added 12 daily trans-Pacific flights to and from Hawaii and 16 interisland flights, said Swan.

“At the peak of the service that we’ve already announced, we will be at 4,900 seats a day,” he said, “That’s 2,800 interisland air seats and 2,100 trans-Pacific air seats daily that have been added to the Hawaii market.”

Those counts don’t include the start of sales that are expected in the “coming weeks” for San Diego, Sacramento and Lihue flights.

Swan said, as with the current offerings, the next Hawaii ticket sales will begin just weeks before the flights begin.

“We usually try to do four to six months of sales (before launching a new route),” he said. “This was unique to us. We’ve never announced or launched service this quickly.”

What will allow Southwest to reach more distant mainland cities is the addition of fuel-efficient 737 MAX 8 aircraft, which Southwest will use to replace its 737-800s later this year or early next year, Hawkins said.

There are 17 cities Southwest serves west of the Rockies that it hasn’t connected Hawaii to yet.

They include San Francisco, Burbank, Los Angeles, Ontario, Long Beach, and Orange County in California; Portland, Ore.; Seattle; Spokane, Wash.; Boise, Idaho; Reno and Las Vegas in Nevada; Phoenix and Tucson, Ariz.; Denver; Albuquerque, N.M.; and Salt Lake City.

According to Boeing, the MAX 8 can seat up to 178 passengers in a standard two-class configuration and fly 3,550 nautical miles, that’s 600 nautical miles farther than Boeing’s previous 737 model. The slightly smaller 737 MAX 7 aircraft extends the maximum flying range by another 300 nautical miles to 3,850 nautical miles.

Southwest has 34 of the MAX 8 jets now and has 212 more on order, Hawkins said. Southwest also has an option on 30 MAX 7 planes, although 23 have been deferred until 2023, he said.

Swan said the MAX 7, which has a slightly longer range than the MAX 8, could open up even farther opportunities, growing connectivity across Southwest’s system.

POSSIBILITY OF GROWTH

Southwest Airlines is adding new aircraft that will allow it to reach Hawaii from the locations listed below. Pictured are a few of the major hubs.

>> In California: San Francisco, Burbank, Los Angeles, Ontario, Long Beach and Orange County.

>> In other states: Portland, Ore.; Seattle; Spokane, Wash.; Boise, Idaho; Reno, Nev.; Las Vegas; Phoenix; Tucson, Ariz.; Denver; Albuquerque, N.M.; and Salt Lake City.