Juliana, this looks like your couple on the Family Search website… https://familysearch.org/pal:/MM9.1.1/FSY1-8YH William ELLIS (age 31), Cheshire, General Labourer & wife Sarah E ELLIS (age 25), with no children, travelled on the ‘Hindostan’ from London to Dunedin, Otago, NZ in 1874.
According to Family Search the ‘Hindostan’ left London on 28 March 1874 and arrived in Otago on 13 May 1874. These dates look odd to me as they suggest a voyage of only 6 weeks. While a steamer could travel from England to NZ in about 6 weeks, the Hindostan was a sail boat and a voyage by sail typically took at least 90 days. However it seems the Hindostan actually arrived in Otago on 13 JULY 1874 (two months AFTER the arrival date on Family Search) and newspaper reports of the time refer to an unusually long voyage of 106 days.
By the way, the NZBDM website https://www.bdmhistoricalrecords.dia.govt.nz/search/ shows Georgina Elizabeth ELLIS, child of William & Sarah Elizabeth ELLIS, was born in NZ on 22 Sept 1874 (just two months after arrival in NZ). BDM Registration No 1875/ 7101.
The BDM Registration No is based on when a birth was ‘registered’ and not when the child was born. So the BDM website says that Georgina Elizabeth was born in 1874, but her birth was not ‘registered’ until sometime in 1875 – thus giving her an 1875 registration number.
To find a child’s actual birth date you need to progressively narrow the start & end dates of your search until finally the start & end dates are the same. This is ‘fiddly’ but it’s worth taking the time to get the correct birth date. The same ‘system’ applies to finding marriage and death dates on the NZBDM website.