Leaders of political parties come and go. Some, the Hawkes, Keatings, Howards, Frasers, achieve some kind of iconic status, for better or worse. Others, the Downers, Lathams, Nelsons, Creans, do not.

But in success or failure, all those named above, and almost all other leaders, for that matter, have one thing in common: They are creature of their parties.

Bob Brown was different. He was one of a select few — think Robert Menzies or Don Chipp — who were their parties. They were not born of their parties; their parties were born of them.

And that is why Senator Brown's announcement that he was stepping down from the leadership of the Greens, and out of the Senate, is a particularly huge deal. In fact, in some ways, it's unprecedented. There were anti-Labor parties before Menzies; Don Chipp was a longtime politician before he started the Australian Democrats out of the left of the Liberal party.

But Bob Brown cut the Greens from whole cloth, as it were. He came from outside politics and turned environmental activism into a political organization.

He didn't do it single-handedly, but it's fair to say that no political party in Australia has been so closely identified with one figure as the Greens are with Bob Brown. He was in from the start of the movement, in Tasmania 40 years ago — he recently attended a 40th anniversary celebration.

He was the face of the campaign to stop the Franklin Dam, over seven years. He was the pioneer in the Tasmanian Parliament, and then in the Federal Parliament, from 1996. And for much of that time he was a lonely presence.

But the party built, and from the time of the last election until Friday, April 13 (unlucky for some), he led the party which held the balance of power in the Australian Parliament.

He was in many ways the perfect leader for the new party, because he played so against type. The Greens's political opponents always portrayed them as a party of radical ideas, which they often were. But how radical did Brown look? He wasn't some wastrel hippie; he was an earnest, country-bred doctor of mild demeanour. Short hair, no drugs, engaging smile, slow drawl and a way with a story or metaphor. He was also a very good politician whose horse-trading abilities were respected even by the hard men of the major parties.

That's why his decision to step down is such a gamble for the party.

The departure of a strong leader is dangerous for any party; for a minor party it can be deadly. Major parties have a protective institutional mass and inertia, which is a strength as well as a weakness. If Labor or the Libs happen to select a couple of dud leaders, they still survive to fight again.

If a minor party does, things can be over very quickly. Look at what happened to the Australian Democrats.

So why would Brown decide to do it now, at the height of his party's success, holding the balance of power in the Australian parliament?

Photo courtesy of the office of Senator Bob Brown

Well, he just reckons it's time.

In his first post-announcement interview, with The Global Mail, he put it thus:

"Well, I've been thinking over the months since the last election. We were away in Senegal at the global Greens conference a fortnight ago. I finally decided that this was it. Paul [his partner] was with me over there. We came home and spoke to Christine [Milne, his deputy and now party leader] and Ben [Oquist, his longtime chief if staff] of course, and then set about how to do it with the least fuss and bother."

He says he's in perfect health, and while the Greens have had their internal ructions of late, there is no evidence that he was pushed.

"The biggest thing that was pushing me was common sense and good reason … I'll be 73 if I stay for the next six years in the Senate. I've watched politics for a long time. I know that some people don't know when to go."

He professes no concern for the future of the party without him. And in typical Brown fashion, he makes his point with a story, harking back to his first election, to the Tasmanian parliament.

"I got elected to Denison on a countback on the day I got out of jail. That was January 5, 1983, after 16 days in Risdon jail, for blockading the Franklin.

"The High Court decision [stopping the dam] came down in July that year. There was enormous hostility in the Tasmanian Parliament. And at the end of that year, the then member for Braddon, later to become Premier, Michael Field said to me at the Christmas Party, 'Well, Bob, you've got nowhere to go now but down.' And I've had that said to me in various forms ever since."

The party will not fail without him, he says, and the success of the Greens, is "patently not" due to him.

"It's the times. You can see it in the parallels in other countries. I've just come back from the global Greens conference. It's spontaneous combustion. It's a global reaction to a materialist age out of hand. And people are very worried for the future."

Successful Greens parties in other countries have changed leaders without it precipitating crisis.

"It hasn't happened in New Zealand, it hasn't happened in Germany. It won't happen here," he says.

Brown portrays himself as just "a product of history", cast into the thick of issues which happened, in the 1970s and 80s, to be starting to resonate with people.

His modesty might be commendable, but that's really not right. Tony Abbott, of all people, clearly showed that with his reaction to the news.

Too often, Abbott said, Bob Brown had "looked like the real Prime Minister of this country".

Abbott sounded almost regretful as he said it; it may be the last time he will get to beat the real Prime Minter, Julia Gillard, with that line. One must doubt that the name of Christine Milne will be as powerful a rhetorical cudgel.

And on the other side of mainstream politics, Labor must also be calculating the meaning of this. What if the Greens vote does decline? Greens preferences were all that kept Labor in office last election.

Or maybe, with "Saint Bob" gone, Labor will regain some of those voters who defected to the Greens over the years.

Photo courtesy of the office of Senator Bob Brown

Photo courtesy of the office of Senator Bob Brown

Photo courtesy of the office of Senator Bob Brown

Photo courtesy of the office of Senator Bob Brown

Put it to Brown that he may just have done Labor a favor by giving them back some of their base, and he is dismissive.

Such questions are imponderable, and would be asked whenever he chose to step aside.

"That's the sort of question which leads people never to get out of politics, to think they're indispensible," he says.

And, as the saying goes, the graveyards of the world are filled with indispensible people.

And so Brown will go on to other things, to a more normal life, but he will retain a role as party elder statesman.

"I will be a Green until the day I die," he said.

"If not for a longtime after that."