The price of air travel may be coming down - but you could be paying extra for snacks or essentials at the airport before your flight.

Some items on sale at Auckland Airport cost more than double their price at the nearby Airport Shopping Centre, and the situation is similar at other major airports.

A 600ml Coke will set you back $4.99 at Whitcoulls stores in Auckland's domestic and international terminals and at Wellington and Christchurch Airports.

At the Countdown supermarket down the road from Auckland Airport the same item costs just $2.78 - about 45 per cent cheaper.

A 750ml bottle of Pump water costs $4.99 at the News Travels store in the Auckland domestic terminal, almost double the Warehouse price of $2.59 and much higher than what you'll pay at Shortland Superette in central Auckland - $3.40.

If you're feeling peckish, don't go for a packet of Tim Tam biscuits at the airport Whitcoulls. At $8.99, it's three times what you'll pay at Countdown and double the Shortland Superette price.

And if you need painkillers, toothpaste or tampons, you'll be paying about double what those items would cost at Countdown or the Warehouse.

Peter Kalan, managing director of Whitcoulls' parent company RedGroup Retail, said the cost of products from the chain's core range is the same at airports as elsewhere.

"Anything outside of the core range will be higher because we buy them in smaller units and don't have the same buying power," said Kalan.

He believed the prices of food and toiletries at airport stores were comparable to a general convenience store or dairy.

At Sumo Salad in Auckland's domestic terminal, a toastie costs $8.90 and a medium salad $11.50. At the chain's Takapuna branch a toastie is $8.20 and a medium salad $10.90.

Paul Woodside, of Spotless Services, who oversees food outlets at Auckland Airport, said the two Sumo stores are owned separately and set their prices independently.

He didn't know why prices were slightly higher at the airport outlet but said it was likely to come down to higher rent.

"Lots of different factors go into it. If we were to open one at Hamilton Airport, for example, different rent would affect the prices."

Auckland Airport spokesman Richard Llewellyn said the airport had a policy that retailers must ensure prices are competitive and not higher than those charged by similar shops in Auckland.

Stores were monitored to ensure they were complying. "We've got a variety of means of monitoring - some is done by the shops themselves and some by us," said Llewellyn. "We do mystery shopping spot-checks.

"It's hard to compare with the buying power of a supermarket - a corner store is probably a better comparison."

Llewellyn said there had been cases of shops charging too much but it wasn't a common occurrence. "We've addressed it with the retailers and they've corrected it."

Monique Oomen said Christchurch Airport had a similar policy. "We encourage retailers to offer competitive high-street pricing and we monitor it."

Wellington Airport spokeswoman Louise Murray said there was a competitive pricing clause in the airport's retail leases and ensuring retailers uphold it would be a focus for the next year.

Murray said the move to enforce the clause wasn't in response to concerns or customer complaints.

"We have many local brands, like Mojo and Fuel coffee, and you'll find their prices are the same as their other outlets around the city."