The photoionisation dynamics of NaK molecules in the (BII)-I-1: state is studied in both frequency and time domain by femtosecond laser spectroscopy. Comparison to model calculations shows that the photoionisation proceeds via intermediate-to-high-n Rydberg states. These Rydberg states act as Franck-Condon windows in the time-dependent experiment, and the molecular ions produced by autoionisation of the Rydberg states are shown to reflect the wave packet evolution in the B state. Consequently, Rydberg state-mediated photoionisation is proposed as a viable method for probing molecular wave packet dynamics.