Harp strings are counted down from the high end of the harp. So, (depending on the harp) the highest strings are in the first or second octave and the lowest strings are in the fifth, sixth or seventh octave.
Pedal harps usually have one or two strings that are higher than the first octave: these are referred to as top octave. A typical pedal harp, with 47 strings will range from top octave G down to seventh octave C: this would be written as G0-C7.
Octaves are counted down from the E string. Middle C is fourth octave C. So, for example, the octave starting on middle C and going up to the C one octave above would be written from middle C upwards as:
C4, D4, E4, F3, G3, A3, B3, C3.