Mr Johnston said the high New Zealand dollar also provided "opportunities for consumers" to get a better deal online from offshore merchants.
Gary Baker, BNZ director of institutional research, said: "The renewed strength partly reflects a bounce back in the growth in online purchases from domestic retailers, following a slow patch over the September-November period."
The index monitored online sales in the categories of fashion and accessories, daily sales, department, furniture/housewares and hardware, electrical and electronics, recreation, books and entertainment media, groceries and liquor, pharmacy, stationery, antiques and flowers. These were estimated to total $2.8 billion, excluding GST.
"Consequently, the online market share of overseas merchants has increased 42 per cent in the month of January, the highest level we have observed so far," Mr Baker said.
Entertainment media grew the fastest, at 25 per cent, followed by furniture/housewares at 22 per cent and groceries and liquor at 18 per cent. But online sales at "daily sales" or "one-day sales" websites declined, following exceptional 2010 and 2011 growth.
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