"Civil unions were really just a prelude to the arrival of same-sex marriages in New Zealand in 2013," she said.
"Some people like civil unions because there is less of a religious element to them but most of these unions don't seem to last any longer than traditional marriages.
"People are people, no matter what they sign or promise. If they want out, they want out ... I'm afraid it is just human nature."
Civil unions were introduced in New Zealand 12 years ago amid a storm of protest from some religious groups.
In the lead-up to the bill being passed, about 5000 people gathered in Wellington for a Destiny Church-organised march against the proposed legislation giving gay relationships legal recognition for the first time.
"I think the civil unions were brought in really to appease the gay community but they have never really taken off," Mungall said. "Gay people thought it wasn't good enough and that they were still being treated as second-class citizens because it did not include the word marriage."
Out of 800 or so ceremonies Mungall has performed down the years, fewer than 10 have been civil unions, she said.
"I expect civil unions will eventually drop off the radar."
In December 2013 award-winning Kiwi musician Anika Moa separated from her civil-union partner Angela Fyfe. They have young boys, Taane Diamond and Barry Kowhai.
Moa and Fyfe became the poster couple for gay marriage equality with a civil union in February 2010.
And two years ago a New Zealand Government official helped a celebrity couple walk down the aisle when their dream wedding seemed to be in peril.
The Department of Internal Affairs customer services team leader stayed at work late to make sure Australian reality TV stars Tresne Middleton and Carly Saunders got their paperwork to marry on this side of the Tasman after a glitch was spotted in the lead-up to their big day.
The duo had previously held an intimate commitment ceremony in 2012.
The popular same-sex pair, contestants on cooking show My Kitchen Rules Australia, wed in New Zealand as Australia does not recognise same-sex marriage.
There has also been a dramatic drop in the number of civil unions since same-sex marriage was introduced.
Civil unions are most popular with women and people born in the 1980s.
In 2012, the year before gay marriages were introduced in New Zealand, 303 civil unions were registered, compared to just 70 this year.
Peter Harrison, President of the Association of Rationalists and Humanists, said despite their declining popularity, he believed civil unions were still a valid option for getting hitched.
"None of the moral harms suggested by its critics at the time have come to pass," he said.
"Civil unions may be superseded by the changes to the Marriages Act, but here again the only case against them is based on moral claims that are not at all supported by the evidence."
