After serious thinking wrote :
Just got home and the circuit the fridge is on has tripped the RCD. On
further analysis it looks like it is the fridge (Fischer and Paykel double
fridge freezer) itself. Any ideas how to figure out what is wrong with it?

Any help appreciated

First you need to make absolutely certain, it is the fridge tripping
the RCD, rather than the domestic wiring or some other faulty appliance
causing the trip. Does the RCD only protect the the one circuit, or is
it a whole house RCD. If it protects just the one circuit, could the
fridge be temporarily put on another circuit, protected by a different
RCD, even if a long extension is needed to do that? Even a plug in RCD
could be used on the different circuit.

If the other RCD trips, then obviously the fridge has a fault. Then
worth taking a look at the back of the fridge, to see if any condensate
water has overflowed the evaporator to drip onto the wiring around the
motor.