I can only guess there is a leak somewhere in that irrigation scheme or maybe there is a leak in the pool.

That water consumption is ridiculous.

and yes I would turn off the irrigation system - no way I would pay 649€ to water the garden. You could try a hose? If you have no access to the pool water system, how do you clean the pool? In any case, there should be a remote tap somewhere on the pool water supply?

Turn on each tap in turn and check to see if the needle moves. This ought to identify which part of the system is leaking.