It’s tantalising to ponder how a film about people who make a living as professional “breaker upperers” – paid by clients to end the relationship with their lovers – might have been handled by a great director of farce, such as Francis Veber or Preston Sturges, or a writer with a sharp and caustic wit, like Tina Fey.

In the hands of co-writers, directors and stars Madeleine Sami and Jackie van Beek – two charismatic and personable New Zealand comedians – the material is not so much witty as rambunctious; the sting of a potentially prickly concept dulled by their innocuous, play-to-the-back-rows approach.

The Breaker Upperers continues a run of crowd-pleasing New Zealand comedies with obvious international appeal, several of which – such as Boy, Hunt for the Wilderpeople and What We Do in the Shadows – were directed by Taika Waititi (the last one with Jemaine Clement). Waititi is executive producer here.

The Breaker Upperers follows Mel (Sami) and Jen (Van Beek), who entered their ethically dubious line of work after discovering they were dating the same two-timing man. For a fee they deliver a relationship coup de grace by any means necessary: through song, by pretending to be outraged former lovers, crashing weddings and special events, and even faking deaths. As the recently dumped Anna (Celia Pacquola) puts it: “You work for weak arseholes who don’t have the guts to talk to their partners.”

Mel and Jen’s golden rule for dealing with clients is “don’t get attached”. This is predictably broken when Mel takes a liking to an 18-year-old rugby player, Jordan (James Rolleston, who played the titular character in the 2010 coming-of-age comedy Boy). This thick-witted fellow doesn’t understand why his girlfriend, Sepa (Ana Scotney), didn’t get the message that he was dumping her; after all, he texted her an emoji of a broken heart followed by a thunder cloud. There are numerous jokes about whether Mel is “screwing a minor”, invariably leading to comments about him in fact being “legal”.

When Mel ogles Jordan through a pair of binoculars, we witness the fantasy in her head: a preposterous slow-mo shot of the young man dowsing his face in Coca-Cola. Moments like this reveal that the film-makers will do virtually anything for a laugh, with little consistency in form, content or internal logic or realism. The Breaker Upperers’ structure is shaggy and the story, at times, close to non-existent. The film unfolds as a loosey-goosey series of comedic sketches – from word salads and trash talking to dance sequences. A riotous karaoke rendition of Céline Dion’s It’s All Coming Back to Me Now demonstrates some flair in the writing – an outrageous flashback used in lieu of exposition.

Sami and Van Beek seem to forget the punchiness of their premise, or even that The Breaker Upperers has a premise at all. The film, while sprinkled with some vaguely risque elements, maintains a tone that is inoffensively cheeky, the comedians’ shits-and-giggles style earmarked by exclamations like, “I’m not wearing any undies!” It all comes down to the execution, which is largely dependent on Sami and Van Beek’s appealing chemistry and sisterly camaraderie.

There are some small but very entertaining supporting performances – including a hilarious turn from Rima Te Wiata as Jen’s sex-obsessed mother – and cameos from New Zealand comedians including Clement. As the sad sack Anna, who encourages the two lead characters to reassess their conscience, the Australian actor Pacquola gives the film its one truly affecting performance; among her achievements is to somehow look poignant while wearing a penis hat.

But The Breaker Upperers is Sami and Van Beek’s show through and through. The film coasts off the energy and rapport of this affable pair, whose smart-mouthed performances are full of pep and fizz. What they lack in wit they compensate for with sheer likability. Part of their appeal is an underlying sense of decency, evident not just in the film’s inevitable moralism but in their personalities and propriety. This plays a part in endearing Sami and Van Beek to the audience, while smoothing the edgy bits of what could have been a punchier, sassier, frothier comedy.