It awarded Ms Smith $6200 in lost wages and a further $5000 because "the dismissal was conducted in such an arbitrary manner it must have negatively impacted Ms Smith and warrants an award of compensation".
The written decision published by the authority says a text from the pool's manager, Warren Webber, on December 13 told Ms Smith she would need to produce a certificate by December 21 otherwise "it ... will be your last day".
Ms Smith renewed her certificate on December 16 but a hard copy could not be printed till the New Year.
Instead she said an email would be sent to her manager advising she had done the training.
"Ms Smith gave a copy of the email to Mr Webber but says he told her, via a telephone call on December 19, the email was not acceptable and she would not be given any more hours at the pool."
In response Ms Smith queried why she would no longer be given hours and asked for written confirmation.
Mr Webber said she would receive it two days later.
"When these events are considered in totality I conclude there can be no conclusion other than the trust decided to cease Ms Smith's employment and advised accordingly more than once. That is a dismissal," the authority said.
It said the only justification being offered by the trust - that Ms Smith did not have the required first aid certificate - failed as a defence.
"It is undermined by the fact the trust engaged Ms Smith without seeking confirmation she had the certificate and was willing to employ her without one for some weeks.
"Second the trust chose to ignore, or at least not investigate further, strong evidence she had renewed the certificate.
"A fair and reasonable employer would not, I conclude, have acted this way."
Secondary to the claim of unfair dismissal Ms Smith claimed she was "unjustifiably disadvantaged" by the trust's failure to take action on her complaint that she had been subject "to workplace bullying in the form of unwanted attention of a sexual nature".
The trust argued that claim had no merit and that it was unaware of the complaint.
The authority agreed with the trust on this point and Ms Smith was unable to produce evidence she had made it.
"In this instance I have accepted the trust was not aware of the claim until after the employment ceased which negates the possibility of a successful claim ..."
The decision was made on May 1 and Ms Smith is yet to receive the $11,200 she was awarded.
When contacted on Friday, Maureen Doyle, who took part on the Employment Relations Authority process on behalf of the trust, would not comment.
"They [the trust] said they don't have any money and couldn't pay me," Ms Smith told the Chronicle.
"In my eye, if the trust can't afford to pay for it, then [Whanganui District Council] should be held liable."
But the council only owns the pool complex.
During the 2014/15 season the trust managed the employment of staff and operations at the pool.
"You can't tell me that council has nothing to do with running the Wanganui East Pool and then you have council staff coming in and checking the pool over," Ms Smith said.
The council said on Friday it was a matter for the Whanganui East Pool Trust.
"The [council] is not involved in this case," a statement said.
"It should be noted that the Wanganui East Pool Trust was responsible for employment decisions at the time of Brooke's dismissal, rather than the Splash Centre management team, which now manages employees at the Whanganui East Pool."