Hi @kalvinder_bains,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
The issue isn't so much with your concrete, nor does the concrete itself really require a repair. The issue is that the soil is eroding out from underneath it, which means surface drainage is the main problem.
The first step is to stop the cause of erosion. Look for things like roof downpipes discharging near the garage, surface water running along the edge of the slab during downpours, or ground sloping toward the slab. Any of these things can contribute to drainage issues, so redirecting downpipes to extend them away from the garage, and regrading soil so water flows away, not toward, the concrete are good steps to take. If you have a good look at how water acts in this area during the next downpour, this will give you a good idea of what you need to look at.
Once drainage is improved, as the void is from the soil eroding, not the concrete eroding, underneath the slab can be filled with a compactable material such as road base. Push the material as far under the slab as possible using a length of timber or steel rod, building it up in layers. Lightly dampen and compact as you go, so it locks together.
After filling, cap the edge to prevent future washout. This can be done by building up a road base sloped away from the slab or using concrete edging along the slab edge. The goal is to seal the edge so water cannot easily get underneath again.
Allow me to tag our helpful members @Nailbag, @R4addZ and @AlanM52 to see what they think and if they have any ideas.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
