Am I missing something?

Chorus came out this morning with its half-year results to December 31 '13. It made a profit after tax of $78 million.

Let's suppose it has a tougher second half of the year, and only nets (say) $50 million. Let's also suppose that all those second half profits get kept as retained earnings, boosting total equity from the $673 million at end December to $723 million at the end of June '14.

Then on these conservative assumptions Chorus's after-tax profit for the full year would be $128 million, making an after-tax return on closing date equity of 17.7%. Using the average equity level during the year (opening equity $624 million, closing equity $723 million, average $673.5 million), the after-tax rate of return would be 19%. Pretty impressive returns for an infrastructural utility company.

So explain to me again: what's wrong with the argument that Chorus is milking the lines business by charging way-above-cost?