An authorised retailer's obligation to ensure that there is a metering installation and that an NMI is assigned for each customer is found in rule 7.2.1 of the National Electricity Rules. That rule only applies in relation to connection points for which the retailer is the Financially Responsible Market Participant (FRMP). While there is a distinction between connection points and child connection points as used in the rules, the glossary states that a child connection point is a connection point unless otherwise specified. That being said, a retailer for a child connection point is not the FRMP for that connection point as it is not part of its market loads. As a result the general obligation of a retailer to ensure that there are NMIs for each customer does not apply in embedded networks. 

Responsibility for the allocation of NMIs rests with the Embedded Network Manager (ENM). The National Electricity Rules specify that the ENM must apply for NMIs for metering installations at those embedded networks for which it holds an appointment. The relevant provisions are extracted below:

The AER's guidance material here: https://www.aer.gov.au/embedded-network-managers supports the above. In practice, an authorised retailer may also be an ENM or will appoint one (per the AER's Network Guidelines).