Greens senator Dorinda Cox has quit the minor party to join Labor in a shock defection less than a month after the federal election.

This afternoon in Perth, the West Australian senator revealed her shift to the government during a joint announcement with Prime Minister Anthony Albanese. She explained that her choice came from “deep reflection,” but she emphasized her dedicated support for the Greens in last month’s election.

“It’s been over a period of time that I’ve considered this,” she said.

“As mentioned, this was no simple decision for me. It involved numerous discussions and advice from various political figures…

“So this wasn’t a decision that I did on the hop or based on emotion.”

Cox had a significant role within the Greens, serving as their spokesperson for several key areas, such as First Nations people, resources, and trade and tourism. She also played a part in choosing Larissa Waters as the new leader of the minor party.

Labor will now hold 29 seats in the upper house to go with its equal-record 94 in the House of Representatives.

While the defection is a major blow for the Greens, who have endured a horror month after losing three MPs including then-leader Adam Bandt at the election, Cox’s defection doesn’t fundamentally change the make-up of the Senate.

The government will still require crossbench or opposition support to pass bills through parliament, and the Greens will still hold the balance of power with 10 senators. 

Greens leader Larissa Waters said while she wished Cox well, she was disappointed with her decision.

“Senator Cox has said that her values align with the Labor Party,” Waters said.

“This is the same Labor Party who this week approved the climate-wrecking North West Shelf gas project, which UNESCO advises will destroy significant First Nations heritage and ancient rock art.

“Senator Cox would have had more chance of effecting change by continuing to work with the Greens in the sole balance of power.”

Cox joined parliament in 2021 when she replaced the retiring Rachel Siewert and won re-election the following year.

She issued an apology last October following reports 20 staff had left her office in the space of three years and that bullying complaints had been made against her.

“As the employer, I take responsibility for any shortcomings in what has occurred during this period and I apologise for the distress this may have caused,” Cox said at the time, although she added the reports were missing “significant” context.