Diners who get fast food sent to their door are paying more for their meal than if they picked it up after "hidden" costs are placed on orders over and above delivery fees according to a consumer watchdog probe.

Consumer NZ says after receiving complaints from customers who noticed the high pricing they now suspect many restaurant businesses - including some of our most popular takeaway chains - are charging more for each item they deliver on top of advertised delivery and service charges.

The watchdog claims the two-tiered menu pricing is widespread across New Zealand's food delivery sector with most customers aware they are paying for delivery, but not knowing exactly how much more it's really costing them.

It plans to lodge a complaint with the Commerce Commission.

"We think businesses should be upfront about additional delivery fees so the customer can make an informed decision," said Consumer NZ chief executive Jon Duffy.

Consumer NZ, in its investigation, said both Pizza Hut and Domino's increased the cost of every item in a customer's order if it was delivered.

It also found KFC, Taco Bell and Carl's Jr also charged more for each item delivered – on top of the advertised delivery fee. The true price of delivery went up with every item added to a customer's cart.

And the practice extended to third-party delivery services such as Uber Eats, DeliverEasy and Menulog with customers paying more for each item than they would if they were dining in.

Consumer NZ found placing a phone order to Pizza Hut to collect a Mega Meat Lovers pizza cost $15.39. But if you opted to get it delivered, the same pizza costed $18.49, plus a delivery charge of $7.99. The true cost of getting that pizza delivered came to $11.09.

It warned that the cost of every item in your cart increased if you opted for delivery.

"This means you could end up paying significantly more for your delivered order than if you opted to collect it," said Consumer NZ.

One Pizza Hutt customer Graham said he got a shock when he mistakenly selected delivery for his online order. He corrected his order to pick-up and was surprised to see the cost of each item in his cart reduce.

Restaurant Brands have been approached for comment.