Voids under house foundation - how to address?

Hi All,

The house is a ~1982 built, single storey, double brick exterior walls, single brick interior walls, hard wood used throughout the roof construction, large number of cement rendered interior walls, tiled & carpeted internal floors. Dimensions of dwelling ~9.2m x 22.2m with approx. 2.5m wide 'verandah' extension of slab on all sides under roof cover.

Location is just inland of Coffs Harbour - very high rainfall area. Soil is clay and rocks just a few cms from the surface all over the property. Site of the building is on the top of a crest/small rise, falling away slightly on 3 of 4 sides.

We purchased the property 7yrs ago - were aware of some of the cracks in the walls & foundation at the time. But didn't give it much consideration due to age of property & other factors. In hindsight something of a mistake but will not assist us now.

There had always been cracks in the foundation of the house, and some cracking in areas of rendered brick walls. These were aesthetic issues only & remain so. However over the years they have in some areas gotten mildly worse, others much the same.

I had asked a number of non-expert persons about what i should do & all including a neighbour felt it was only due to soil and rigid materials of house - so felt it was a non-issue. Clearly wrong, I knew this but ignorance is bliss sometimes.

Not too long ago - in applying an under slab termite perimeter barrier (drilling 10mm holes around entire perimeter of the brick walls, 10mm from wall at 150mm intervals for injection of fipronil solution) - I found that in many areas there was significant voids under the concrete slab.

Being a complete novice I thought this quite unusual - but focused on what was already a large job I didn't give it too much thought. However upon hindsight I realised this was almost certainly the cause of the foundation cracking and wall damage to the property.

Thus here I am.

I will note I am again near certain that these sub-foundation voids were caused almost exclusively by terrible drainage around the slabs edge after heavy rain. The large 14m x 27m roof had only a single downpipe on each corner and water would regularly exit the 'weep holes' from the quad style guttering as they overflowed during even mild rain. It was not unusual for significant pooling to occur on 3 sides after heavy rain and soil depression had occured in these spots making this even worse.

I will note that I am certain that trees & animals are not a factor in the slab issues - the nearest instrusive tree (Liquidamber, is 35m from slab).

To combat this I reinforced all the guttering and added an additional 4 downpipes, also adding extra soil to fill the depressions and tried to build up these areas into inclines away from the slab edges.

But the voids under the slab remain. So how to address?

I am unsure whether to go with 'mudjacking' or resin/polyurethane injection? Unsure if the latter is available in this non-metro area. As with many folks at this time, money is somewhat tight so I'd try and negate costs by doing as much of the work as I could but recognise that theres a limit to this in such matters.

I should have taken care of this earlier - but would appreciate advice or thoughts to move forward with atleast ensuring no future damage occurs.

Please let me know if additional info is required and thank you in advance for your assistance.

PS. Tried to directly upload images - but failed twice so have posted linked to Google gallery of them - apologies.
https://photos.app.goo.gl/Ve63FHgbpkBD3S8f7