Brown will step down from his Senate seat in June. He has held the seat for 16 years.
At a press conference this afternoon, Brown said his party was growing, and was on a trajectory to form government in future.
The Greens currently hold the balance of power in the Senate, and have been instrumental in the implementation of a carbon tax.
A new deputy leader will be announced later today.
The Conversation spoke with University of Adelaide’s professor of politics Carol Johnson about what this decision will mean for the Greens party.
Is it possible to imagine the Greens without Bob Brown?
They are certainly going to face some challenges because Bob Brown was a force of unity in the party. One of the potential challenges they’ll face with his resignation is that ideological tensions between various wings of the Greens may become more apparent.
For example between some members of the far left who tend to emphasise socialist issues, and people who see the main priorities as environmental issues.
The other challenge they’ll face is that they really don’t have anyone who is as good a communicator as Bob Brown. Over many years, Brown has learnt how to pitch the Greens' message at mainstream Australia.
He learned to do this many years ago when he was an environmental campaigner in Tasmania. He would be out in the wilderness looking very grubby and dirty, camping out there. But he would always have a clean suit and tie, and before the media turned up he’d change into them and look very respectable.
So he is a master at pitching the Greens' message to the ordinary Australian voter.
The Greens don’t really have anyone who is quite as good at doing that and certainly Christine Milne doesn’t have either the charisma or the ability to cut through with a clear message like Bob Brown.
What has been Brown’s legacy for Australian politics?
Bob Brown really was one of those pioneering Greens politicians. There’s a number internationally, but he’s one of the ones who has survived best.
He has gone on to lead a greens party for many years and he has made the Greens a viable third force in Australian politics.
Of course, one always has to be careful about assuming that minor parties are going to continue to prosper, because there is a long history in Australian politics of them facing problems.
But certainly for the immediate future the Greens seem likely to continue to be a significant force in Australian politics. And in particular, they are at present important because of Labor’s minority government.
What will happen if, as is likely, we have a Liberal government next time around with a significant majority will be another issue.
What challenges face Christine Milne and what kind of leader will she be?
I’m not sure Christine Milne has the skills to keep the different sections of the Greens together and to provide a unified face for the Greens. But we’ll have to see.
There are also issues about whether her negotiating skills are quite as good as they should be. And whether some times she’s too inflexible.
For example, she was responsible for a lot of the Greens' negotiations with the Labor party over the Carbon Pollution Reduction Scheme (CPRS). And it’s very interesting to speculate what situation Australian politics would be in today if the Greens had passed the CPRS.
One of the key challenges if the Liberals get in, will be if Tony Abbott tries to repeal the carbon tax. And what happens if he tries to repeal the mining tax?
A lot will depend on if they still have the balance of power. A lot will depend on whether Labor, if it’s comprehensively defeated at the next election, decides to vote with the Liberals or the Greens.
But there’s also other potential dilemmas, if Tony Abbott simply decides not to collect the carbon tax or if he explores other constitutional options if he can’t repeal the legislation.
What do you make of the timing, politically?
Clearly, Bob Brown wouldn’t have wanted to do a John Howard. He wouldn’t have wanted stay on too long as leader.
He would have wanted to leave plenty of time for his successor to get their image established with the electorate.
There have been some rumours about leadership tensions within the Greens. And of course, there was a recent speech Bob Brown gave that was ridiculed by much of the mainstream media.
So it’s hard to know precisely what was happening behind the scenes. Certainly some of the Greens at the press conference, such as Sarah Hanson-Young and Adam Bandt, looked pretty shell-shocked by the announcement.
We’re not quite sure what was happening with some other members of the Greens. And one of Christine Milne’s challenges will be how to handle [NSW Greens senator] Lee Rhiannon.