Sainsbury's closed its online store for two days earlier this month as store IT problems prevented its shops from being able to process orders for delivery.
The supermarket takes orders on its website but these are routed to stores near customers, which process the orders, pick items and deliver them.
Sainsbury's chose to stop access to its e-commerce site after stores could no longer process online orders.
The news follows the completion of a major project to bring Sainsbury's IT back in house. In 2005, Sainsbury's began the project after cancelling a £1.7 billion (NZ$4.4 billion) outsourcing deal with Accenture. A thorough operational review had concluded this would improve technology operations and costs."
Sainsbury's website resumed full service at 6pm on a Thursday, after problems emerged late on the preceding Tuesday. The company said it would not make deliveries until Saturday, but added that the website had been "thoroughly tested".
A spokesperson at Sainsbury's says the company stopped website sales to avoid a backlog of orders. Customers whose orders were not processed have not been charged, and have received a £10 voucher, the spokesperson says.
Sainsbury's says it had "not attributed a cost" to the website being closed. But it says the site normally has 8,000 to 10,000 customers a day. Some observers have speculated that Sainsbury's has lost more than £1 million in online orders over the two day period.
In its latest results, Sainsbury's chief executive Justin King said the company's online operation was "continuing to perform well", with sales growth at a solid 40% year-on-year in the quarter to June 14.
Robin Goad, analyst at competitive intelligence website Hitwise UK, says the two days offline would have a "huge impact" on Sainsbury's, even though its online sales were smaller than those of Tesco, the Uk's largest supermarket.
Some 8% of visitors to Sainsbury's website went straight on to visit Tesco.com during the two day closure, Goad says. "On a normal day, 1.5% switch from Sainsbury's site to Tesco's. That means there was more than a fourfold increase on the days its website was down."
Macro 4, a performance management software supplier, says the situation was "typical of many large organisations who are not able to take a rigorous enough approach to testing web and business software systems". Macro 4 says many businesses had different departments testing different parts of systems. Small changes to software functionality could easily cause major problems.
Rob Steggles, marketing director at hosting firm NTT Europe Online, says the cost to Sainsburys would be significant.
"Many companies will have a complete back-up version of a site which can be turned on automatically, but having this can be very difficult for a site as complex and advanced as Sainsburys.com," he says.
"There are a number of factors [that can cause] a site to crash, from heavy traffic, an error with back-end software and systems to human error from someone updating or modernising the site."
In a statement, Sainsbury's apologised for the problems.
"We are pleased to advise customers that our home delivery website is now up and running. It went live last night at 6pm has been accepting new orders and amendments to existing orders. Deliveries will resume from 8am on Saturday morning," the statement said.
"We experienced a technical glitch which affected our ability to process orders. We wanted to ensure that the site was restored to normal and had undergone vigorous testing before reactivating the site. In this way, we can ensure that customers only receive a service from us that we are happy with."