Ive done the Youtube and Google thing, last resort is here!

Where we moved early this year has a grass lawn. Its brown underneath. Green grass is thin, as in like someone who is balding. So Ive been trying to work out whats wrong. Here is my progress to date

Thatch.

Seemed likely as the brown stuff is not loose its dead grass. So, thatch will block sun and air on the topsoil and be a barrier of sorts to water. Bought a thatching rake, and got half a green bin off the front lawn which is worse than the back lawn. Not all of it had much thatch though. But now, the thatch is really thin after detailed checking out back. And its not packed solid, so it can't be that.

The grass that decides to migrate off the lawn into the garden grows great, so again that points to the lawn itself.

The soil is about 8 inches thick then its hard clay, packed down tight. Whatever packing measurement they use its supposed to be 120 somethings, but here it was 200. (house is 5 years old, I have all the documents when the owner built it) I would have thought this would cause lack of drainage if anything. And when it rains a decent amount (ChCh) the back patio floods a little, and behind the garden shed, indicating poor drainage. But the gardens which are mainly shrubs and a few flowers grow great. So that confuses me.

Compaction.

The ground seems hard. But unsure how to gauge that as I don't often check that :-)  nor poke a thin knife into the lawn as I did last night (searching for the 5 underground sprinklers, found 3 !) Im thinking compaction is an option, stopping moisture getting deep. The lawn is getting quite a few weeds so that isn't great, maybe due to unhealthy grass, or more likely, thinned grass. 

Insecticides

I read excessive use can reduce organisms in the ground, that reduces aeration and soil cultivation by them

Nitrogen

I read that excessive nitrogen makes the grass grow fast, too fast, and it becomes unhealthy

My next step is to buy a good sharp fork and punch holes in the earth and allow moisture in. Also to help find the other 2 underground sprinklers. On that, how deep might the hoses be, as I dont want to puncture them. The heads are an inch below top soil. I will add lawn fertiliser that I bought last weekend. 

Right now Im thinking aeration, NPK fertiliser, and heaps of water

Any other hints?