Customers of TrustPower can pay their bills and examine previous payments online, while "key account" customers can transfer figures into their own accounting systems.
The electricity generator and retailer has had a Web site for about three years.
The system for smaller customers works through New Zealand Post's eBill, but key account customers have access to a separate database.
"I put together the initial system as a prototype," says IT administrator Ross Mahon, "then took it to a developer [the Advantage Group], who put it on the Web."
The system is based on Microsoft IIS products, with a link to Trustpower's back-end systems. It is essential to maintain this link in real time, so information on the state of a customer's account with TrustPower is always up to date.
"We have a billing system written in a Pick derivative, so getting information out of that posed some difficulties."
But even the most basic aspects of an e-commerce site, like setting up a Web site that will talk to all browsers in the market, has its difficulties, he says.
Using e-commerce in its strict sense, it has been a minor aspect of TrustPower's venture on to the Web.
"I would say 85% of the content is informational and 15% transactional,"says sales and marketing manager Keith Tempest.
The new business created from e-commerce and Web information, however, is practically zero to date, because greater forces are at work in the market making it difficult for customers to switch suppliers, he says.
"Perhaps that's a bit too pessimistic," he reflects. "We really have no way of measuring whether a new customer was attracted to us partly because of our Web site."
The billing help is aimed at improving the relationship with existing customers, and this helps to keep them loyal.
The metropolitan customers are, not surprisingly, keener on the electronic interface, as they are with manual direct debit.
"We're doing well in the markets we've approached directly, but the Web is currently a very small part of our operation. That will change, though, as consumers get more used to transacting business electronically.
"The big business-to-business drive of the near future will again be informational rather than transactional. "A key issue with corporates is information on their consumption."
This will be provided electronically "within months", Tempest says.
The development will enable a business to benchmark its own consumption against a scatter diagram of other companies of the same scale in the same business.