A few things can cause this to happen.
1. The unit is not level, and the evaporator water is not travelling to the drain properly. It is fine to have the 'front' of the fridge higher than rear (up to 5mm), so try extending the 2 x front feet a little.

2. The drain is blocked or 'frozen'. This can happen for different reasons, the drain hole is located behind the rear inner wall in fridge so you don't have eye contact etc. which makes it hard to diagnose. The easiest way to see if blocked is to pull unit out and remove rear bottom cover, this will give you access to the drip tray. A small pipe should be promiment there, grab it and 'blow' air strongly to see if it is obstructed etc. To check if it's frozen over you need to remove the inside rear wall, which means you need to fully empty the fridge, take out shelves and runners (long bits that hold shelves) and remove rear wall.

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