Couple of possibilities here... 1. Trying to emulate what GNE...

Couple of possibilities here...
1. Trying to emulate what GNE and others in NZ are doing very successfully - basically have households entire/most by the wire/pipe services tied up in one account - gas, electricity, data (reduces customer churn).... AGL already had gas and electricity... and plenty of cash which they did not know what to do with - hence Southern Telco buy... AGL then try and bundle purchased Southern telco customers onto AGL electricity & gas. (They also purchased an electricity retailer to get their customers in the same way - Click Energy)
2. With entry into Southern Telco one could then progress to data centers.. which are also power intensive businesses (great for AGL), which are also highly in demand at present. Data centers have fiber optic connections - most telcos (such as Southern) have their own network of fibre typically between capital cities etc to minimize third party network charges. You then sell your commercial customers onto your own data centre which has the fastest connection to the data centre as all on the same fibre... this is where things fell over 12? months ago...

Other point is that telecommunication routes have been largely disrupted on the retail side by NBN. Price ceilings have been placed on speed / data, which before used to provide telcos with some fat cream for people.

On the electricity transmission front, power lines and poles - it is largely still in the hands of state government owned entities... or has been privatised by government bodies and then either sold to private equity or into other businesses such as SKI (Spark Infrastructure)... might also point out - transmission is also a highly regulated market on pricing (set by National Energy Regulator) and I think they review pricing yearly or so? https://www.aer.gov.au/system/files/TN-Transmission%20Pricing%20Fact%20Sheet.pdf is an example.

Happy to be corrected here... but just trying to answer your question