"The student was vulnerable, and at the time the intimate relationship commenced Student A would have been in his last year of school or just finished it, had he not been in prison."
It found that the age difference between them was relevant as it "tends to accentuate the power imbalance".
The teacher told the tribunal after its decision that she could not afford to pay the standard proportion of half of its costs, which totalled $46,832.
She said her only income was $238 a week in family tax credits and an accommodation supplement of $42 a week.
Her partner, the former student, earned $35,000 a year.
"We have no money to spare and get some support from the church, family and friends," she told the tribunal.
However tribunal deputy chairman Nicholas Chisnall ruled that he was "not satisfied that the evidence adduced by Teacher C demonstrates the type of financial hardship that requires a reduction in the quantum of the order", and ordered her to pay half the costs - $23,416.
"It is ultimately a matter for the [Education] Council to decide whether, and how, to pursue the debt," he said.