Hans also had a son named 'Hans' named here as Hans Christian Oestmann
Carsten had a son named Hermann Oestmann
Hermann had three children Minna Dora and Otto. Dora Oestmann, though female, is the ancestor of the Oestmann's in Australia.
Dora Oestmann was partnered to an "August Storm" and they had a child Rudi. Mr Storm died in the first world war. Rudi, his son, decided to take on his mothers name 'Oestmann'. So, the name 'Oestmann' continued with all the decendents who travelled to Australia in the 1958.
See more specifics of the Oestmanns in Australia here: