Honda Jazz wont take water in the water fill cap?

I recently purchased a Honda jazz 2005 mode from a garage, top condition. I'm going on a long journey tomorrow and routinely like I would with any car I checked the water and oil levels. (cold engine). Water level was halfway between min and max so I attempted to top it up with coolant mix, however, as in poured the h20/coolant mixture into the water tank it just came straight back up as though the tank was full, although I can see it's only half full. Any ideas?

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