But what is the essence of the interdependency relationship?

The essence is the relationship is the concept of a “close and personal relationship” which relationship would normally involve “living together” and the provision of “support” by one to the other (whether financial, domestic or personal care).

In the context of a parent-child relationship evidence must be provided which shows that:

• the parent and child provided emotional support for each other;
• the parent and child contributed to the costs of running the household or contributed to the costs of acquiring or maintaining household property;
• the parent and child attended various functions together – for example family functions, entertainment functions;
• there was financial support from at one to the other – being support of a regular and material nature which support constituted a significant part of the recipient’s standard of living;
• there was domestic support and personal care from one to the other – such as cleaning, cooking, attending to medical appointments;
• the child was not merely staying with the parent on a stop-gap or temporary basis; and  
• that the relationship was not a temporary arrangement or for a fixed period.

As with any evidence, merely asserting (for example) that care and support was provided is not likely to convince a disinterested third party (such as the ATO, a court or a trustee).  What is required are instances of care and support which was provided – eg the child taking the parent to medical appointments.  The instances should be reasonably detailed and frequently occurring.