When?

[Gletterz]

What time of year should I chop these at the bottom to become bonsais

 

[Bonsaidoorguy]

Not much to go on here.

 

[Gletterz]

I’m in Australia. I’ve got a sango kaku osakazuki crimson wave and autropurpeum
And shishigashira

 

[rollwithak]

Do phones down under have cameras on them yet?? Pictures are worth a thousand words.

 

[ShimpakuBonsai]

So you are talking about Japanese maples but what is your goal with the chop?
Do you want the branches you cut off the tree to become a bonsai (making cuttings with it)?
A little bit more info on your goal with the tree and some pictures would be nice.

Some more info on Japanese Maples can be found on the bonsai4me website.
Link: https://bonsai4me.com/speciesguides/acer-palmatum-varieties-japanese-maples/

Pruning Unless extension is required to form new areas of foliage or branches, cut new growth back to one or two pairs of leaves following bursts of growth through the growing season. For trees that require short internodes, buds can be pinched out leaving 2 new opposing leaves as soon as they are visible, this will produce fine, twiggy growth with short internodes and also promote back-budding. Remove all growth with long internodes.

Leaf-cutting can be carried out in mid-summer, this results in smaller leaves, better ramification and stronger autumn leaf colour. However, defoliation should only be carried out on healthy trees, never in the same year as repotting, never 2 years running and never on weaker red-leaved varieties.

For highly ramified Japanese Maples, ensure that enough light can get into the canopy by removing one of each pair of leaves.

Hard-pruning and formative pruning should be carried out in Autumn after leaf-fall (preferably within 1 week) or during the mid-summer semi-dormant period when wounds can heal very quickly. Never prune during Spring as all Acer species have a habit of bleeding profusely which can severely weaken the plant or even result in the loss of branches. Ensure all wounds are sealed.

For trees that are displayed for their bare winter silhouettes, summer pruning might be considered.

 

[Shibui]

Are these Japanese maples grafted? If so chopping low could completely remove the cultivar and you'll end up with a plain Japanese maple.

Now should be a good time to chop maples in most of Aus. Any time after the leaves are well open is usually OK.

Risk is less if the trees are young but gradually increases as the trunks get thicker and older.

You can see it is not quite as simple as you may have thought. Lots of ifs and buts associated with many bonsai techniques so we need lots of info to narrow down the possibilities.
You are likely to get much better advice if you narrow down your location a bit more. Many species react different in different climates and timing will vary quite substantially too as Aus is a bloody big place with a wide range of climates.