Hi @Jona,
I am a Registered Migration Agent with Fair Go Visa Consultants.
If you are applying without being married or having your relationship registered (for a 820 or 309 Visa) you will need to provide 12 months of de facto evidence at the time that you apply (so you cannot lodge and then get 12 months of evidence from that point, it needs to be before).
If you are married, it will waive this requirement. However, you will still need to show that you are in a genuine and continuing relationship. You cannot just apply with a marriage certificate. You will still need evidence that goes back sufficiently far to demonstrate your relationship and address all four of the relationship aspects - financial, social, nature of the household and nature of commitment.
I hope this helps.
info@fairgovisa.com.au
https://fairgovisa.com.au/