Despite his actions, Mcdonald denied any wrongdoing, saying he was drunk at the time and had gone into the changing room by mistake.
At an earlier court appearance lawyer for the defence Richard Leith said there was no SIM card in the phone.
However, Judge Philip Crayton said he had to sentence Mcdonald on the summary of facts he pleaded guilty to, which stated "the defendant removed the phone's memory card".
"The phone was analysed by the digital forensic unit and images were recovered. The memory card was not recovered," Judge Crayton said.
"The extent of what might have been revealed had he not got rid of the SIM card is concerning. That obstruction is an aggravating factor."
The images of the middle-aged woman were clear, while those of the children were said to be out of focus, but indecent and objectionable.
Leith compared Mcdonald's situation to that of a Peeping Tom.
"In this case the intent wasn't to produce this for passing on to others. It was just some photos that were taken on his phone," Leith said.
"It's no different than the Peeping Tom looking into the bedroom, watching someone get changed. It's of that nature, this is the modern technology equivalent of it."
Judge Crayton disagreed.
"No, no, this is someone who's clearly taken them for two reasons. The first is his own gratification, the second is for the gratification of others," the Judge said.
"This is not a Peeping Tom case. To conceal ones self sufficiently, to have the confidence to do so and to be thought through the way that it has been, takes it beyond that."
He said that in the case of a Peeping Tom, the perpetrator often observes at a distance and the victim has the option of closing the curtains.
Whereas on this occasion, children had elected to change in the privacy of a cubicle.
"There would be absolutely no thought, let alone expectation that their privacy would be intruded upon," Judge Crayton said.
"They expected to be ensured privacy and the defendant had in a considered way, and surreptitiously, circumvented that."
Mcdonald pleaded guilty to two charges of making an intimate visual recording and knowingly making or copying an objectionable publication.
Judge Crayton sentenced him to 14-and-a-half months' imprisonment with six months' end sentence conditions in Whanganui District Court on Wednesday.
Mcdonald has an extensive history of offending dating back to 2012 when he was convicted of assault with intent to injure and assaulting a female, among other charges.
He is undergoing treatment for mental health issues in custody, where it was discovered that he suffers from anxiety, causing sleep deprivation and insomnia.
Post-sentence, Mcdonald will require the approval of a probation officer to be in the company of people under 16, to possess or use an electronic device or to be within 20 metres of a place where children congregate.