A customer has been left with the sh*ts after an angry baker delivered a cake shaped like, well, poo.

New Zealand's Emma McDonald has refused to apologise to 24-year-old Micaela Harris, whose relative had ordered an engagement cake from McDonald's business, Oh Cakes.

But frustrated by a negative attitude from her client, McDonald reaped her revenge by sending a "turd cake" to the party on December 20 with the message "eat sh*t" - and it didn't go down well.

"We r (sic) just at my sister's engagement and got your cake, we and every1 else is absolutely disgusted," read a text from the relative to McDonald.

According to The New Zealand Herald, the customer had expressed annoyance after McDonald was forced to reschedule a meeting. On the flip side, McDonald says the client did not provide enough insight about her requirements over the cake.

"Give me some ideas?? Oh wait you have none apart from wanting chocolate. I have a brilliant idea for your cake!!! - so here it is, your turd cake! Hope you learn your lesson," she posted on Facebook.

"I have no regrets at all about what I did. I feel she got what she deserved."

It's now escalated so far that the Mayor of Invercargill, Tim Shadbolt, has expressed his disappointment about the incident.

"I think this must have been done with humour, but sometimes humour can go terribly wrong," he said.

After receiving a mountain of criticism on Facebook, McDonald detailed her charitable work - giving away cakes and clothes to the needy - on her personal page.

"I give one lady a piece of my mind and everyone thinks we are bad people. Well I don't think we are."

The cake is reminiscent of a scene from the 2011 hit movie The Help, when maid Minny serves a slice of revenge to her nasty employer, Hilly, in the form of a poo cake.

News_Rich_Media: The Help - Eat My Shit