Updated: Oct 10
Autumn is by far the best time of year to plant. A statement that would have been widely disagreed with thirty years ago, but changing climate has turned ideas around. When planted in September and October, plants have time to grow roots into the still-warm soil before winter arrives. With spring planting, they have very few weeks to grow roots before the inevitable April/May drought.
If you can establish plants before winter, they will also flower better the following year.
There are, of course, exceptions to this. Anything vaguely tender, certain shrubs such as Pittosporum, and those grasses that are warm weather growers, such as Miscanthus. These should wait until April, except for tender plants which need to go in after any frosts have passed.