National Australia Bank's (NAB) latest online retail sales index showed online spending was up 14 per cent to $13.9 billion in the 12 months to June.
This is the second month that online sales have weakened, after growth slowed to 18 per cent or $13.7 billion in the year to May, compared to a 24 per cent rise in the year to April.
NAB Group chief economist Alan Oster said the fall is indicative of consumer attitudes.
"I suspect consumers are being cautious," he said on Thursday.
"Whilst 14 per cent is a hell of a lot better than what traditional bricks and mortar are doing, it's dropped a lot."
Mr Oster said he believed the slowdown was caused by a drop in big-ticket items like furniture and electronics.
"I think what we're seeing is a consumer who's reluctant to spend and a bit nervous."
Online sales now represent 6.2 per cent of traditional retail spending.
The Australian Bureau of Statistics will release retail spending figures for June on Monday.