You need to buy Australian birth / marriage / death certificates for the family back to the first immigrant, starting with your husband's father. His birth cert will have his parents' names and other details, their marriage / deaths will have their parents, etc. Australian certs are usually very detailed and will show how long the earliest immigrant had been here, where he came from, and, depending on which state, details of his parents and their residence. Only after you have all these details is there any point looking for IoM or other overseas records.

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