He painted a portrait of a simple life, saying they walked along the beach and cooked together.
"Well, I'd cook and she'd wash up. Just dinner and relaxing and laughing," he said.
Hewitt had an affair with the princess when he was an officer in the British army. At the time rumours were circulating that Prince Charles was also having an affair with his current wife, Camilla Parker-Bowles.
He and Diana were forced apart when Hewitt was deployed to serve in the Gulf War.
They parted for good when the affair was exposed in the media.
Since then rumours have circulated that Prince Harry was Hewitt's son after media speculated on the physical resemblance between the two.
"It sells papers," he said. "It's worse for him [Harry] probably, poor chap."
Prince Harry's colouring led to speculation that Prince Charles wasn't his biological father.
Sunday Night also interviewed Diana's bodyguard, Ken Wharfe, who slept on a camp bed in a hallway outside the bedroom the princess shared with her lover.
Wharfe said: "It was pretty obvious what was happening [but that] it wasn't for me to moralise on this."
He went on: "I didn't really have a view on it openly or inwardly, you know. It was her life, their life. I was there for one specific reason, I would never let her down."
The Sunday Night show also featured an interview with Diana's former butler, Paul Burrell.
Princess Diana, laughs in this televison image, during an interview aired on the BBC Panorama programme when she admitted the affair with Hewitt.
Burrell claimed to be in possession of a letter from the princess which he received from her in the months before her death in a car crash in Paris in 1997 and in which, he said, she prophesied her own death.
Quoting from the letter, Burrell said: "This particular phase in my life is the most dangerous. My husband is planning 'an accident' in my car, brake failure and serious head injury in order to make the path clear for him to marry Camilla."
Burrell said that the fears expressed in the letter came to her after she visited clairvoyants.
"Some of these thoughts did come from mystics but you can't get away from the fact that she wrote it. She took time to sit down and actually write down her fears."
