Ok, there are quite a lot of low-mid level herbal painkillers that are quite legal and fairly easily available.
Ru Xiang and Mo Yao (frankincense and myrrh) are great, especially for injury type pains. Can be dissolved in alcohol (both are plant resins) and applied. You can also drink them, but the taste is very harshly bitter, and they irritate the stomach. I very rarely prescribe them for patients for internal use for that reason. Wholesale, I pay less than $40 for a kilo, which would last a darn long time indeed. The mix of the two is better than twice the dose of one.
Yu Jin, and Yan hu suo are better if you want to actually have something to drink that will strengthen, be reasonably palatable (ish). Quite cheap, and effective for various skeleto-muscular pains, and a few others as well. These take ordering, but you can get them nice and easily off the net or at a herbal supplier in a major city.
Strengthening the back so you don't actually get the pain is the preferred route though, my preferred strategy is to 'treat root and branch simultaneously', stopping pain and fixing the issue causing it. Du Zhong ( Cortex Eucommiae, note that I'm speaking 'herbalist' and not 'botanist' here) and Tu Su Zi ( Semen Cuscutae, 'Chinese Dodder seeds'), in combination can work well here.
If the disk needs healing (in TCM terms, the yin is also deficient), then add Shu Di Huang (Radix Rehmanniae Preparata). This is easily obtainable in a larger Chinese grocery.
Du Zhong and Shu Di Huang can be had from many Chinese groceries, Tu Su Zi is a little harder to get.
There's a vast array of herbs that might actually treat a 'bad back', but a lot of them are a bit dependent on what has gone wrong (and your constitution in general), to reccomend here. These are all pretty safe and easy. Daily dose of the above herbs would normally be in the 6-15 gram range.
Hard herbs like these are best boiled in a pot with a lid for half an hour.
https://chineseherbswholesaler.com.au/en/products
The cheapest if you're going to actually be in Melbourne, they finally got a little showroom.
http://www.chineseherbsonline.com.au/
Another Chinese herb supplier, with a showroom, but the internet order actually works for most things.
https://www.australherbs.com.au/
Austral herbs, order by net. They have a minimum $50 order, and stock is western herb oriented.
For best results, see a T.C.M. professional and get a diagnosis and a script, which you can fill yourself if it contains common stuff.
All herbs mentioned are fully legal and available in Australia.... Sometimes I think this forum needs a good guide to herbal painkillers, anxiety and depression treatments and the like.