Well-placed sources tell New Idea that while the TV hosts have spent mornings smiling happily on screen and appearing as close, personal friends, it was a very different story when the cameras stopped rolling.
And it seems the source of that simmering tension was Karl's shocking secret - that he was paid substantially more than Lisa throughout their 10-year working relationship.
'It was always a bone of contention,' says a source. 'Lisa suspected she was underpaid compared to Karl from day dot, and that was upsetting.'
In a New Idea exclusive it can also be revealed that Lisa had grown tired of Karl's behaviour in recent months and that relations between them had become increasingly strained.
The 57-year-old-mum-of-three - who's been inundated with support since walking away from Channel Nine - has further told friends she feels 'betrayed' by Karl's lack of support during her battle to gain equal pay.
Well placed industry sources go on to tell New Idea tensions have been growing in recent months. While Lisa was initially supportive of Karl after his controversial split from wife of 21 year Cassandra Thorburn last September, Lisa grew tired of his antics after he went public with Jasmine.
The final straw was when Lisa reportedly saw secret research showing she was 'carrying Karl', following viewer backlash over his divorce, sparking her to demand the same $2 million pay packet as her male co-host. But that was a move Karl refused to support.
'People see them as one of the same and assume they are best pals,' a Nine Network source tells New Idea. 'They are no such thing.'
Since the star's resignation - and subsequent defection to Channel Ten, where she'll star as a host of The Project in 2018 - Channel Nine have slammed Lisa's demands for equal pay, with the network's CEO claiming that Karl is 'worth more' than Lisa.
Karl, meanwhile, suffered public backlash when he complimented Lisa's ability to do housework and then come to work every morning - leading critics to call him 'sexist.'
For the full story see this weeks issue of New Idea.