Pulsed field de Haas-van Alphen measurements of the Gamma centred part of the Fermi surface in ZrZn2 are presented for fields up to 35T. The spin-split Fermi surface was found to be field dependent. It is argued that the observed de Haas-van Alphen frequency is different from the frequency defined by the Onsager relation. An expression is derived based on the Stoner-Wohlfarth model which describes the difference in spin-up and spin-down frequencies as a function of field strength. The observed difference in the effective mass for the two spin directions can also be explained by this model and it compares well with band structure calculations. An anomalous behaviour of the second harmonic of the majority surface de Haas-van Alphen frequency was observed. A search for other frequencies was carried out but in spite of the high sensitivity, in terms of m*TD, no other orbits were seen.