According to Google Des was quite well reguarded in the Oxford United youth setup, I've read he was manager of the first team in NZ press releases but nothing elsewhere, he's also listed as the 1st team manager for the Wellington Phoenix since 2015 on his Linked In profile (rather than assistant manager), so it might be an english thing to talk up your CV somewhat (like including age group results with your national team results, at least Des doesn't have his own Wikipedia page to maintain).
It's pretty disappointing that NZ coaches haven't gotten their diploma and shows a lack of ambition, apparently a caretaker can only run the team for a few months without the Diploma so if a new coach isn't found before the end of the season Greenacre will have to step down and let Des run things.