The company said in a statement on Tuesday that the transaction was due to be settled on Thursday (3 December) and would mature on 3 December 2025.
In May, the NBN Co borrowed $6.1 billion in debt finance from external markets.
The Federal Government, which has funded the building of the network, flagged some time ago that it expected the firm to pursue debt financing through external markets, including in order to start the process of re-financing its loan.
That has now blown out to $57 billion, with the government doing a backflip on its plan and reverting to the Labor plan of full fibre for most premises.
In September, the NBN Co said it would make fibre to the premises available to about 75% of homes on the network, making a total of eight million in all, by the end of 2023.