Can a mother's partner begin a period of shared parental leave while the mother is still on maternity leave?

Yes, if the mother has provided her employer with a maternity leave curtailment notice, and all the relevant eligibility criteria and notice requirements are satisfied, the mother's partner can begin a period of shared parental leave while the mother is still on maternity leave.

For example, the mother could submit a leave curtailment notice stating that her maternity leave will end three months from the date of the notice. The mother's partner can take shared parental leave (with at least eight weeks' notice) at any time after the mother has submitted her leave curtailment notice; he or she does not have to wait until those three months have passed.