The 5p charge on plastic bags which came into force on Monday should be more, according to an environmental campaigner.
Brian Wilson, former leader of the Green Party in the Assembly, suggested the charge, introduced on Monday morning, should be three times higher at 15p.
"I believe the ultimate success of the legislation will be when plastic bags are phased out completely. I do not believe the 5p levy will achieve this," the ex-North Down MLA argued.
"The aim should not be to raise revenue; it should be to change consumer behaviour, but I welcome the fact the revenue will be used for environmental schemes."
The Northern Ireland Independent Retail Trade Association (NIIRTA) meanwhile voiced its own concern over the scheme but promised to implement it.
Chief executive Glyn Roberts said: "We share the objective of reducing the amount of carrier bags in circulation and that end up in landfill sites. But we believe that sensible changes to the scheme, such as capping it at 5p, letting retailers distribute the levy proceeds to local charities as is the case in Wales and dropping some of the ridiculously high fines should be made.
"We are concerned that the collecting and administration of the levy may pose a real burden on our members and that during the recession."
There will be exemptions for takeaway hot food and drinks, prescriptions, unpackaged food and uncooked meat to protect safety and hygiene. Retailers are required to pass the proceeds of the levy to the DoE. More than 1,000 businesses have completed their online registration.
Environment Minister Alex Attwood insisted on the need to curb the 30,000 carrier bags thought to be used every hour.
"People tell me they are concerned about climate change and want to find ways to make personal, family and local contributions to addressing the threat. The levy is precisely this," he argued.
"This levy is intended to help protect the environment by dramatically cutting the number of bags used," he said.
"Working with the retail sector we are aiming for a reduction of at least 80%. Some retailers have already indicated that they will be eliminating single use bags.
"Consumers don't have to pay the levy. They can simply bring their own bags when shopping," the minister stressed.
Northern Ireland is the second UK region to introduce the charge after Wales, although a decade has passed since the measure was introduced in the Republic.
Which bags are exempt from the charge?
Where and when will it be charged?
Question: So what is the Carrier Bag Levy?
Answer: It is a new environmental measure that requires shops and retail outlets in Northern Ireland to charge for each new single-use carrier bag they dispense.
Question: When is the levy being brought in and how much is it?
Answer: From Monday – and 5pm for each new single-use bag.
Question: Why is the levy being introduced?
Answer: NI gets through around 250m carrier bags each year – about 140 per person – and needs to cut that number dramatically to reduce environmental damage.
Question: What harm do plastic bags do?
Answer: Many wild animals and even farm animals have died a very painful death after digesting a plastic bag. They create a litter problem as bags throughout our countryside, often caught in trees and hedgerows and denting the promotion of tourism.
Question: Where is the money going to and what will it be used for?
Answer: Retailers must pay the proceeds of the 5p levy to the DOE. The money will be used to help fund environmental programmes and activities.
Question: Will the levy apply to just single-use plastic bags?
Answer: No. The levy will also apply to single-use bags made from paper and other natural materials.
Question: Where will the levy be charged?
Answer: The levy will be charged in all shops and retail outlets that dispense new single-use bags
Question: And what about online shopping?
Answer: If you buy online from a retailer distributing the goods from premises within Northern Ireland (eg grocery shopping), then the single bags used to deliver your goods will be liable for the charge.