Former Australian Greens senator Dorinda Cox has accused the party of racism and ignoring serious claims she raised.

In her resignation letter, reported on by the ABC, the Yamatji-Noongar senator for Western Australia denied allegations of bullying levelled against her, and said she had suffered an "unremitting" campaign of bullying herself over the past 18 months.

Senator Cox, a former police officer, defected to the ALP last week, claiming her values were more aligned with the government.

She had previously been scathing in her criticism of Labor on a range of issues, including climate change, justice and foreign policy.

In her formal resignation letter, the ABC reports Senator Cox said she was no longer bound by confidentiality obligations, and could put on the record how the Greens "failed [her] as its last First Nations MP, and continue to fail First Nations people".

"In my experience, the Greens tolerate a culture that permits violence against First Nations women within its structures. In this respect, the party is deeply racist," she wrote.

At least three of the complaints lodged against Senator Cox were made by First Nations women: Senator Lidia Thorpe, former ACT candidate Tjanara Goreng Goreng, and Mardathoonera Elder Aunty Esther Montgomery.

More than 20 people resigned from Senator Cox's office, which employs a staff of five, in her first three years in the job. Many of them accused the senator of bullying.

"My reports to the party and parliamentary workplace services range from being assaulted at Perth Airport by a party member, which was ignored (indeed, I was advised not to report it to police), to incidents involving my staff who were isolated by the state and other MPs offices," Senator Cox wrote in her letter.

Senator Cox and Dr Goreng Goreng reportedly made complaints against each other to WA Police in 2023 after an altercation at Perth Airport over the Indigenous Voice to Parliament.

Dr Goreng Goreng denied she harassed or assaulted the senator, asserting she only touched Senator Cox on the forearm to say hello, noting that the interaction was witnessed by an Elder who was with her.

The office of Greens leader Senator Larissa Waters told the ABC they only learned of Senator Cox's resignation letter after being contacted by the national broadcaster.

In a statement, a Greens spokesperson told the ABC: "These claims are disappointing, unrepresentative of the support Senator Cox received, and ignore the substantive work undertaken by the party to find a resolution to the complaints made both by and against Senator Cox, and to address the breakdown in her relationship with Greens' First Nations members."

"The Greens are an anti-racism party, and pushing a senator to take complaints seriously is not bullying," the spokesperson said.

"As the [Independent Parliamentary Standards Commission] and [Parliamentary Workplace Support Service] are the bodies created by parliament to address complaints from staff, they can continue to investigate ongoing matters. This is unchanged by the senator's decision to move to a party that continues to destroy First Nations cultural history through approving coal and gas projects."