Kitchen plumbing issue...

[PlateSpinnerJuggler]

So - tonight I could hear water running - thought "the dishwashers loud or has someone left a tap on" yet to find water gushing under my sink...

I've since noticed an access to my waste pipe which is open... (pic below)

My assumption is that there is a block somewhere along the line causing the waste water to back up and out this opening.

Firstly I've only lived with the newly plumbed house for less than two months - so how can it be so blocked.

But also why is this pipe open in the first place? Shouldn't a blockage block up back through the sink plugs not pour through a giant overflow pipe?

Keen to hear from anyone in the know...

House newly renovated - clearly builders and their plumber will be getting calls and videos in the morning...
So frustrating!!!!

Can you show all the plumbing?
Is that opening where the water was coming from?

Yes that opening is where water was coming out.
Dishwasher was on at time and then when I tested sink and waste disposal it leaks no matter what's on... I'll take another pic shortly when I'm downstairs.
It's not possible to see all the pipes as some are behind the carcass.
So frustrating.
It's def linked to the waste but is higher up than all the other "out" points that go to it (sorry if not explaining well) will take more pics later

So pics below... it's not super clear but basically it's triple sink with waste disposal in middle.
The leaking pipe is just behind the waste disposal and I've run my hand along it as much as I can behind the carcas and it leads to the vertical pipe behind the right hand sink which goes under the tiles - so assume the waste out (sorry don't know correct terminology)