I got started the easy way: I wrote a story and send it to a national magazine, and it was accepted. Several hundred stories and a dozen books later, I still wonder what exactly urged me on me at the time.

Well, John
I would say you were in a minority to have been accepted by a national on your first attempt. I started by doing freebies (for the Master Gardener Association) in the local press, then used them as clips to get myself into other jobs and positions. Clips, whether local of national can always be useful. I would also caution you that in Australia, you have a large continent (as is the USA) stick to the growing conditions you know. If you try to write as a continent-wide authority, you will doubtless come across as generic and useless as if I were to write for California or Florida, when all my gardening has been in the UK and midwest.

Katy:
I was VERY fortunate.

Kath,
Just start writing, and at the same time start exploring outlets for your work. Don't think of it as a necessarily linear process  first free, then paid outlets  but look into everything. It's usually easier to break into regional publications than it is national, but bottom line is you should write about what you know.

Is there a local paper that does not have a weekly garden column? A regional magazine that includes home-style articles? If you don't find an outlet right away, write anyway. Give yourself an assignment of writing one seasonally appropriate garden article a month. Then when you zoom in on a publication you will have several examples of your work. In my experience, there is no better way to improve than to write regularly.

I have written several articles just because the subject interested me some have been published and some have not, but published or not, I benefited from the writing of the piece. Not all seeds germinate, but if you don't sow the seeds you'll never find out what you can grow.
best of luck, C.L.