A United Airlines flight stranded in Canberra for eight hours en route from San Francisco has finally landed in Sydney.

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The UA863 flight was destined for Sydney but was diverted to Canberra because of debris on the runway at Mascot.

The plane arrived in Canberra at 9:00am (AEDT) but passengers were unable to leave the tarmac because customs and immigration facilities at the domestic airport were not able to process them.

"Until a request is received for formal border clearance, Customs and Border Protection is providing an area outside the aircraft in which passengers can get fresh air and stretch their legs without formally clearing the border," a spokesman from Customs and Border Protection said.

The spokesman said it was up to the airline to decide whether passengers could disembark from a plane.

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They were temporarily let off the plane and allowed to walk around and have refreshments, before being asked to reboard.

The plane left Canberra bound for Sydney about 5:00pm.

The 20 hours passengers had spent on board the plane included the flight and time on the ground in Canberra.

Some passengers who took a connecting flight from New York to San Francisco said they had already been on a plane for 30 hours.

Passengers said they were told they would be stuck on the tarmac while they waited for two United Airlines pilots to arrive on a Qantas plane in order to fly them back to Sydney.

The United Airlines pilots ran out of flying hours after the non-stop leg from San Francisco.

A United Airlines spokeswoman told the ABC the passengers were due to be flown back to Sydney at 4:20pm.

By that time passengers left at 5:00pm, they had been delayed by eight hours.

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The experience was particularly frustrating for one passenger, Fabio Calabria, who had originally planned to travel to Canberra after landing in Sydney.

He will now have to return to Sydney to be processed by customs, and then drive back to the ACT.

"In Canberra but not allowed off the plane. Have to fly back to Sydney the drive back here," he tweeted from the runway, earlier today.

Passengers said the bathrooms on the plane had run out of toilet paper and soap.

Jerry Constantine from Chicago said a message came over the loudspeaker about 12:40pm advising passengers they would be let off the plane for a while.

He said the plane felt quite stuffy and during the four hours the passengers could not be let off.

A flight attendant opened a door to let in some fresh air.

He said most people on board were calm but annoyed, with elderly and young passengers the most distressed.

Mr Constantine said the pilot had apologised profusely and criticised the local authorities in Canberra for not being flexible and understanding.

Another passenger, David Defelice, said the airline staff handled the situation well.

"It's been a long day... I'm very sunburnt from being on the tarmac for four hours this afternoon," he said.

"We're good, the crew was good. They took good care of us, despite the situation."

