Your question lacks a bit of detail. Looking at Google Images, it appears that the sink tap is also the shower head.
Where exactly is the water appearing? If not from the tap itself presumably the leak is under the sink where the hose connectors and shower are connected to the tap assembly. You need to stick your head under there while it is leaking. I assume there is a cupboard door or removable panel for access to the underside of the taps.
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