The former chief executive launched an explosive attack on the prime minister and Australia Post chairman Lucio Di Bartolomeo at a Senate hearing on Tuesday.
Ms Holgate controversially departed Australia Post after Mr Morrison angrily condemned her over a luxury watches scandal in parliament.
"I lost my job, a job that I loved, because I was humiliated by our prime minister for committing no offence and then bullied by my chairman," she told the committee.
She said Mr Di Bartolomeo unlawfully stood her down under the direction of the prime minister, making her leadership untenable and threatening her health.
"I have done no wrong. Their bullying of me was far from over. I was subjected to a biased investigation and intimidated with constant threats of further allegations and criticism," Ms Holgate said.
"Throughout this time, my health had deteriorated to the point where I could not find my voice to fight back.
"This is a day the chairman of Australia Post and the other men involved in what happened to me will be held to account."
Ms Holgate said she was speaking out to prevent anyone suffering workplace harassment and bullying.
"The simple truth is I was bullied out of my job. I was humiliated and driven to despair," she said.
I was thrown under the bus by the chairman of Australia Post, to curry favour with his political masters. But I'm still here and I'm stronger for surviving it.
Mr Di Bartolomeo is due to respond when he gives evidence to the Senate committee later on Tuesday.
Ms Holgate quit under immense pressure after it emerged she gifted four luxury watches to executives as a reward for signing a lucrative deal.
The Morrison government announced Woolworths supply chain boss Paul Graham as her replacement one day before the much-anticipated hearing.