Compatibility summary
You will need a travel adapter in Niue; plug types used there differ from Spain.
Plug comparison
Spain plug types: C F
Niue plug types: I
There is no common plug type between Spain (C F) and Niue (I). You need a travel adapter.
Voltage comparison
Spain: 230V, 50Hz
Niue: 230V, 50Hz
Voltage difference is within a range many devices tolerate; check your appliance label.