Animal behaviour scientists say the conduct of the pair, kept at the Aileron Roadhouse in the Northern Territory, is rare and surprising.
The owner of the roadhouse, Greg Dick, told the BBC that the animals spent a lot of time together and had "been in love for a while".
Experts say the "aberrant" behaviour developed due to years in captivity.
Mr Dick said he had seen the kangaroo sleeping with, cuddling and "carrying on too busy" with the female pig, named Apples.
"Tourists often come and say the pig and kangaroo, they were doing rude things," he said.
The kangaroo, which is named for its prodigious libido, has been at the roadhouse (an Australian fuel stop and shop) for 28 years.
Sydney resident Ryan Frazer photographed the intimate behaviour of the pair during a recent trip through the region.
"We noticed they were touchy, even when they were standing side-by-side," he said.
"Then they were hanging out and nuzzling, and the pig jumped on the back of the kangaroo for a bit."
Mark Eldridge, principal research scientist at the Australian Museum, said the behaviour of the kangaroo was aberrant.
"Hand-raised kangaroos or kangaroos that are habituated to people and other animals can display atypical behaviours," he said.
"I know other cases where hand-raised kangaroos and wallabies ... don't think of themselves as kangaroos anymore and see other animals as potential sexual partners."
Dr Eldridge said he was aware of a case where a wallaby repeatedly tried to mate with rabbits it was locked in a cage with.
Derek Spielman, an animal behaviour expert at the University of Sydney, said female pigs emitted a very strong pheromone, which could explain the kangaroo's attraction.
He said that female pigs in heat were "programmed" to stand still when pressure was applied to their lower back, which might explain Apples' acceptance of the kangaroo's advances.
"Once they've got a relationship, she'll probably stand there for the kangaroo at any stage, if they feel confident and comfortable with each other," Dr Spielman said.
"It's unusual, you don't often see it, but that's because it's an unusual situation. These are two species that are on their own, and their only option is to befriend another type of animal."
A pig and kangaroo kept in a captivity in northern Australia appear to have formed a rather close physical bond.