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I own a Mazda Atenza SW, AWD. It has been great and normally I am getting 450km per tank of 91 fuel (inner city running). The last couple of times I filled up, I am only managing to get 400km if lucky. This increase in fuel usage concerns me as I have not changed my driving style.
The car is due for a service next month. the engine is still performing as normal, nothing out of the ordinary. Should I be looking at anything in particular such as fuel system or air, before taking it into the mechanic??

You don't say how many kms the car has travelled, but as it's almost due for a service, it could be something as simple as the spark plugs nearing the end of their life or the air filter becoming blocked.
While the fuel useage is more than before, it's not hugely more to suggest a particular fault with the car. Also, as it's still performing OK and presumably not blowing black smoke, which could indicate running rich (perhaps due to a temperature sensor).
As a thought, I'm presuming the running you are doing is allowing the engine to properly warm up. Short trips in cold weather can mean the engine isn't getting up to temperature, so will affect fuel consumption.
The best advice is to mention it to your mechanic when you get it serviced - but as I said, it may just be spark plugs or the air filter.