The court heard the "Bindi House" was completed in 1983 when the principal complaint was 18.
Harris said the alleged victim came to stay in the main house at that time and she felt excluded.
Harris, then in his early 50s, told the court he took her a cup of tea in the morning and she grabbed his elbow and indicated he should sit on the bed. She then kicked off the duvet revealing a bare leg.
"It seemed to me she was being very flirtatious with me," Harris told the court, adding it was flattering to have a young lady show such interest.
Harris touched the teenager's leg - "his heart thumping like mad" - but then got up and left the bedroom.
"I find it very hard to discuss this, it's highly embarrassing," he said.
"I was a married man, she was a much younger girl, I shouldn't have been doing it."
When the teenager visited again another time things went much further, the court heard.
"She seemed very, very flirtatious, coquettish, looking at me with some sort of sexual chemistry."
This time he took her a cup of tea as an excuse to visit her in the guest bedroom. He says he kissed her, there was foreplay and then he performed oral sex on her.
Asked by defence lawyer Sonia Woodley QC if she was a willing participant the Australian said: "Yes she was, definitely."
The same thing happened on a subsequent visit and Harris told the jury: "She seemed to be welcoming the whole business and enjoying it."
On another occasion the pair went for a walk down a secluded lane and the alleged victim started rubbing the celebrity's crotch. She then performed oral sex on him, the court was told.
Asked if she was a willing participant Harris stated: "She started it."
Once when both he and the complainant where staying with Bindi in Devon the entertainer crept into her room naked "sure some sexual activity was about to take place" after she'd fondled his crotch earlier.
The artist and singer told the court the pair tried to have sex but he prematurely ejaculated "which was very depressing".
He said the relationship "ground to a halt" in the mid-1990s after the woman move to Norfolk.
Around the same Harris and his wife offered the Bindi House rent free to a woman in her mid-30s.
Harris said she was down on her luck and "she wasn't a well girl".
She became his driver and, the entertainer told the jury, gradually their relationship became physical with "sexual intimacy".
Harris said he didn't feel good about the second affair and Alwen was "rightly" devastated when she found out.
Earlier, Harris admitted he was a very "touchy feely" person but insisted he didn't assault his daughter's friend when she was 13 and joined the Harris family on holiday.
Asked about the main complainant's allegation he indecently assaulted her after she had a shower at their Hawaii hotel in 1978 the entertainer told the court: "No, it didn't happen."
He also denied having assaulted her after wrapping her in a towel on the beach after she'd come out of the ocean and when the family holiday continued in Australia.
Earlier, the star imitated the sound of his famous wobble board, sung a verse of Jake the Peg and explained how a didgeridoo was made as Ms Woodley ran through Harris's career highlights.
Harris is charged with indecently assaulting four girls in the UK between 1968 and 1986. He denies all 12 counts.
- AAP
