Whenever an applicant has to present a public document of one country to the authorities of another, the document has to be legalised in its country of origin.
Why do I have to get my documents legalised?
The reason for the requirement of legalisation is quite simple. If you have to show the Spanish administration that you and your husband are married and your marriage certificate is issued by the Australian authorities, how can Spain know that it is legitimate? Spanish authorities do not have access to Australian official registries or documents. Thus, for the Spanish office to accept the certificate as true and legitimate, it has to be certified as such in Australia.
What is the Apostille?
To make this process easier, many countries around the world have signed the The Hague Apostille Convention in 1961. Members of this convention can certify documents for their use in other member countries by attaching the Apostille. Find the official list of member countries here.
Countries that are not members of this convention have their own ways to certify documents for their presentation in other countries.