There are at least 109 people listed on passenger lists into Australia via Fremantle 1919-1922 with the surname JOHNSTON and 218 as JOHNSON at the National Archives of Australia's website.
It is possible that the 1921 birth that Maddy has found is for a person who may be still alive, and therefore that person is entitled to privacy of the individual, which in NSW extends to their actual names.
As it is more than 100 years since the birth, the record is available for any member of the general public to purchase anonymously. It will include information about her parents, their origins, ages, occupation/s, and the nee name of baby's mum.
It is entirely possible that the baby, born in NSW, is NOT connected in any way to our OP's quest. The death cert for David should give the number of years he had lived in NSW and in any other state. That then helps to determine how to find David and Agnes on inbound passenger lists. An official transcription is an effective but cheaper option to obtain the details on the NSW D.C. It should give the names and ages of the children of the marriage, when and where married and other info that will help validate the information our OP already has obtained.
JM.