How interesting! So:
24 June 1869 Bazett Watkins and Agnes McEwan marry at her mother's house in Auckland, NZ
By 26 October 1869, they had separated (according to the newspaper notice)
30 Mar 1870 their son, Percy Hervey, was born.
[I think the name Hervey probably comes from a relation of Bazett's. Bazett's older cousin, Mary Ann Delamotte, had married Charles Robert West Hervey. CRW Hervey was a colonel in the army by then. Bazett's family tended to use surnames of relatives. Perhaps they had had a brief reconciliation when the child was born, or Bazett had registered the child.]
16 October 1870 Agnes (who was still married to Bazett) married Edward Shattock, using the false name of Burns, to avoid being exposed as a bigamist.
They emigrated to Australia, changing the name of Percy Hervey to Edward Foster. They also altered their marriage date, so that Edward appeared to be their child.