Where in australia are you? Which direction are they facing? How old are they if you planted them or if not any estimates if they came with the property? What's on the other side of the fence? Is it your yard or someone else's? Are the areas level or is one higher than the other? Are they the lp's growing above the fenceline (cos it looks very dense) or are those trees on the other side coming over and if so what are they?

sorry for the 20 questions, but let's get a "picture" of the whole problem

I'm in Sydney.

The plants are facing East (as in, the fence is running N-S)

No idea on age - they were well established (and healthy) when I moved in 6 years ago.

It's my side of the property (hence the fence) and the plants extend to about 20 cm above the fence line which slopes down consistently to the street. This is the level I've kept it trimmed at.

There is only 1 tree of note and it's not interfering at all with the hedge.

It's rather a narrow planting area but if they've been going OK for all these years, that would rule out a lot of things. And there are enough past years where weather extremes have probably been experienced. Barring some sort of accidental poisoning.... something in the nature of a defoliant, or something used on the driveway that's washed in, I can only think, as you did, that the burial in chip mulch set them back.

Maybe the chips leached something of sap into the soil, or generated too much heat buildup, or simply smothered surface roots.

I can't suggest anything other than TLC and a bit of fertilizer to see if you can get them going again. I don't know enough about pruning to comment further.