Ms Jackson exposed the corruption within the union's Sydney base, implicating senior officials Craig Thomson and former HSU general secretary Michael Williamson, who is now in prison.
She told the Royal Commission into union corruption that the decision to turn on her colleagues was a difficult and conflicted one to make.
"I was friends with these people. When you work in the union movement, you make friendships. It's a community," she said.
She said she first suspected something was not right after a work meeting at Williamson's holiday house.
"It dawned upon me after that visit, seeing what I saw and seeing some the behaviour of the people there, that something had to be done. But I knew that if I did something, it wasn't an easy thing to do, because they'd be after me."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Ms Jackson also told the inquiry that she hired a private detective to look into alleged irregularities at the Health Services Union because she didn't think police would take her suspicions seriously without good evidence.
Ms Jackson told the commission that before hiring a private investigator she went to union Secretary Gerard Hayes in 2011 with the evidence she had, and was warned her career could be destroyed if she pursued the matter.
"He to my surprise basically said to me that he knew there was corruption but there was nothing he could do about it because Mr Williamson was much too powerful and that he would destroy both him and I," she said. "And we should just wait until he retires. Once he retires we can make Williamson CEO of the organisation and then everything would be ok."
Ms Jackson has been accused of using $1 million in union funds to pay off two personal credit cards.
She maintains they were used for work expenses.
Seven witnesses, including Ms Jackson's ally and fellow whistleblower Marco Bolano have already appeared before commissioner Dyson Heydon.
The commission has so far heard about intimidation used by senior HSU officials against whistleblowers and rivals.
