If it's too good to be true it probably is -- this is a lesson Kiwis who have tried to buy the latest iPhone for $1 are learning the hard way.

An online offer which has gone viral on social media platforms has left New Zealanders hundreds of dollars out of pocket after they unwittingly signed up to a monthly $139 subscription in the belief they were buying an iPhone 6s for $1.

NetSafe spokesperson Chris Hails said 15 complaints had been received since online advertisements appeared earlier this month. Half of those who complained had already been charged.

"People have seen those adverts and have clicked through. A number of people have entered their credit card details."

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Hails said punters had fallen foul by not reading the fine print properly and entering their credit card details, unwittingly signing up for a subscription.

"Most of us don't ever read the small print, so most people would go 'oh fantastic' and enter their personal and banking details, then find out they have been charged money on their cards," he said.

"[This scheme] has had a big marketing campaign. People have reported adverts on various websites and platforms. They also had a big campaign on Facebook a couple of weeks back."

Hails said those who had entered their details on the site were unlikely to get a refund from their bank because they had agreed to the terms and conditions.

Those who had been caught out should contact their banks and get their credit cards cancelled, he said.