This just popped up in my flowerbed. Fast growing, long slender bright green leaves. Any ideas on what it is? Thanks.
Pokeweed, Phytolacca americana.
It is definitely a pokeweed. and may grow to nine feet tall, with large, alternate leaves and a carrotlike taproot. It may become a very invasive weed in gardens and is difficult to eradicate when it becomes well-established.
A native plant (in the US, not sure about Australia, SamWright,) that most gardeners find too big, bearing too many seeds, to be welcome in a tended garden, perennial also as mentioned. Large beautiful, soft leaves, wild red stems... People often post here saying it looks great, until they're told it's a weed.
If you'd like to experience it, just chop it down before it drops ripe seeds in the fall, you'll have a lot of fodder for the compost pile or bin. Then pour boiling water on the stump/root to kill that part, slow enough that it dribbles deeply and does not spread sideways to the roots of desirable neighboring plants. No digging necessary. Looks like there are no other plants within a foot or so, from the small area visible in the pic, or you could just pull it up, should be easy at that size.
Pokeweed, "Phytolacca americana L", is a perennial that is quite easily spread by the seeds in produces if allowed to grow. Birds are the primary distribution vector.
In early growth stages the plant is edible, kind of, and the American Cancer Society is researching the plant as a possible cancer treatment.
To control keep the above ground growth cut if you do not see it early enough to pull the root out.