cheapest option would be to DIY replace the treated pine sleepers without any other changes

Thanks for your suggestion @oklouise. Unfortunately we aren’t super handy, not sure if this is something we can manage!

Replace timber "sleepers " with concrete sleepers

Include drainage behind the new wall

Not Super Handy ?

Start with a small section of wall,

There would have be a Utube video.

If the walls are less than a metre and not providing structural support for buildings then try a handyman or gardener

Yes agree with Kate, a handyman/gardener could be a perfect solution for you.

How close to the street curbing is this, could you consider just removing the old timber and rake the soil towards the street and plant ground covering plants beneath the current hedge plants, another photo looking back to wards the steps from the right might help.

The ground covering that I was thinking of is an Australian Native which also has a bush species, and is quite robust. Julie H will most likely know what it is

Lomandra is good for stabilising slopes.

Siriuskey, are you thinking Hardenbergia?

Jacqui, are you saving for more extensive future work on the property or looking to move later?

Best solutions might differ.

If selling, five years worth of plant growth is worth more than a cheap structure.

Landscaping is DIY friendly if budget is the priority and a low retaining wall is very doable. From what I can see of yours, I'd do two low walls rather than one higher one. Whatever product you choose, it should come with information on how high you can build it without additional engineering.

Do you want a hedge for privacy? Is there a reason you need to remove the existing hedge and start again?

Dichondra Silver Falls is the trailing native that looks great as an under plant. Acacia Cousin It is also attractive but a bit more bulky.

macyjean, the pennies just dropped it's a Grevillea

Great idea siriuskey, these could look fabulous, Grevillea ‘Gin Gin Gem’ is a great ground cover, Grevillia’carpet layer’ is great also absolutely love dichondra’silver falls’ leucophyta ‘silver nugget’ just a few Aussie beauties.

I think Grevilleas are very good at stabilizing banks and being Australian native are easy to manage water wise, it would certainly be a cheaper idea and within 5yrs would be very established

Thanks for all the suggestions! A bit more info: the wall is about 1m high, maybe slightly higher and for support a level garden above it. Below it is a bit more the. A metre wide strip to the street. We do like a hedge both for privacy and also safety - to prevent kids falling off the edge .
I don’t think we’d be able to get away with sweeping it to the street and having a slope- I think the land does the to be supported. So retaining wall might be the way to go.

I'd identify the hedge plants and seek advice on how to keep them. Growing a new hedge would take time. Some plants tolerate things like root disturbance and changes in pH due to concreting better than others.

Modular blocks are not difficult to DIY, getting the foundation right is important but after that it's just assembling it. Or there's sleeper walls but with concrete sleepers so they won't rot again. Ensure there's drainage behind. The only thing is if it's getting above 1m it needs more structural strength. For example one type only needs a level base and some sand for a low wall but needs a concrete footing above 1m. Some aren't suitable for higher walls. You need to look at the specs and installation instructions for each.