Establishments can't delete reviews from many of the sites, even when businesses change hands.
Online reviews of various Wanganui eateries revealed diner frustrations about poor food, bad hygiene and aggressive service.
"I will not talk about the steaks as even thinking about the grill top murder committed at [restaurant] causes me physical pain," one customer lamented.
"Someone had left a number 2 in the toilet which was not cleaned up; demonstrating a lack of toilet checks, (sic)" posted another.
One diner vented about alleged threats from the restaurant chef.
"[Staff] assertively stated the amount of the bill and expected payment and said that they had met f*****s like us before ... Next an enraged chef waved a steak knife inches away from my face and shouted abuse. This abuse escalated to an invitation to step outside and settle it. (sic)"
Restaurant Association NZ national president Mike Egan said online review sites had grown "hugely" in recent years.
But concerns around the legitimacy of some reviews was pushing people back to word of mouth as a more reliable source.
"There are so many fake reviews. I read a review the other day where someone claimed the [side dishes] were deep fried in butter. You actually can't deep fry in butter because you can't get it up to that temperature."
It was more helpful for both parties if a complaint was lodged at the time, so the restaurant experience could be put right, Mr Egan said.
"You go to America and you go to a restaurant not to your liking ... you say, 'My steak's too well done', and you get it fixed," he said. "In New Zealand we have a bad experience, don't say a single thing and then get home and anonymously post an online review."
There was a tendency by some to abuse the sites, so reviews should be taken with a grain of salt, he said.
"If we were to become held hostage to all of the reviews and react every time ... you wouldn't know what to do."
Owner of Palmerston North cafe The Tomato, James Pettengill, caused a stir defending his establishment online last month.
"Before you take your opportunity to soil our good name with your petty little contrived review, you may like to first look at our menu before you cook up your fake review ... Go jump in the lake!" he wrote to one customer's post.
Another response, in which he reviewed a "menopausal" customer, was similarly blistering.
Mr Pettengill has been accused of lashing out unfairly at his customers, but argued he had a right to reply.
Last year's American Express Dining Survey found more than a third of New Zealanders who used social media to rate their dining experience posted online within hours of the meal.
The survey also found those who dined out the most frequently were the most influenced by online restaurant reviews.
