The school therefore advanced him $4097.22 "with the expectation that Mr Simons would repay the money when he received his due entitlements".
Running alongside the pay issue was a personal grievance Mr Simons raised in November 2009, and mediation was held on February 24, 2010.
The outcome of that was the school would pay him compensation, the amount of which was not specified in today's finding, with a "full and final" settlement signed on April 22.
Union delegate Gavin Kay told the authority he suggested to Mr Kay that he deduct the $4097.22 from the amount he was awarded but that Mr Simons "very firmly" rejected the possibility.
The school tried to withhold the compensation when Mr Simons continued to refuse to repay the salary advance but he then successfully sought a compliance order from the ERA.
ERA member Ken Anderson said in today's finding he had "considerable empathy" for the school but it missed its opportunity to have the issue addressed before it signed what was considered a "full and final" settlement.
"While I share the view that Mr Simons obtained an unjust enrichment by refusing to repay the moneys owed, regrettably the school missed its opportunity to obtain repayment when it entered into the record of settlement," Mr Anderson said.
"While I find that Mr Simons' stance on all of this is morally reprehensible, I have to say that the school is legally bound by the terms of the record of settlement and the matter cannot be relitigated."
Rotorua Girls' High School board of trustees chairwoman Kathy O'Donnell told APNZ Mr Simons was one of two teachers the school had given a pay advance; the other had repaid the money as soon as the ministry reimbursed her.
The school was decile three, meaning its students came from a low socio-economic area; the money Mr Simons had failed to repay meant students who already had little would suffer, she said.
However, she could not rule out advancing a teacher's pay should the situation ever repeat itself as the school had a duty to be a good employer.
"I guess the only way for the school to protect itself, if we ever found ourselves in that situation again, is simply to say 'the cheque is there but you don't uplift it until you have signed the piece of paper (saying you'll repay it),'' Ms O'Donnell said.