Locating a leak.

[Pim]

I have a leak in the roof or siding of my IP. I discovered water on the timbers behind the wallboard above a window a while back. There is water damage to the sill where the rain comes in (where?), runs down the timbers, accumulates above the window then seeps through and drips onto the sill. It has only happened twice since I've been here but the evidence shows it has been happening for years.
Whom do I call to locate the source of this leak and to test in other areas to see if the water is running elsewhere. I am pretty sure it is. A plumber? A roofing specialist? A builder? Some other expert?
I'd like to call someone today as last night it happened so there will be fresh evidence. I am in Dunedin if anyone has a recommendation.
Thanks

[TheLiberalLeft]

Might be flashing above the window or roof where the ingress trickles to that point (hard to spot). I'd go for builder and/or roofer.

[Maccachic]

A Building surveyor should be able to identify it. http://www.buildingsurveyors.co.nz/

[Pim]

Wow, quick replies. Thanks

[Gladdynook]

A builder or surveyor will be able to locate it, but I'd get a plumber or roofer as he'll be able to repair it also.

[skid]

Especially an ole 30+ exp. builder

[Davo36]

Yeah get an old experienced plumber.

But if you want to have a go yourself, use a flashlight up there while it's leaking. The water shows up really well.