If the relationship between the borrowers is not spousal, then most banks won’t allow a third party to be brought onto the loan to assist with serviceability, if they are not a joint owner in the property.
These are requirements are the same for parents and children.
Most banks would require each party have at least a 20% ownership share in the property in order for their income to be used to assist with serviceability.
We have access to one lender who on a case by case basis will allow a 1% ownership share, which is great if the third party is worried about land tax or future capital gains implications.