VODAFONE customers' bills could rise by as much as $200 a year after the company announced it would be charging for data in "blocks" rather than actual usage.

The telco also said it would no longer provide free access to social media sites from next month, massively increasing data consumption and forcing prepaid customers to upgrade their plans.

The change means each visit to a site or use of an app on a smartphone will be billed as one megabyte - no matter how long the "session" is and how much data is used.

The move by Vodafone goes against the actions of its competitors.

Optus recently downgraded its billing block from 10 megabytes to one megabyte to now even charging customers per single kilobyte - one thousandth of a megabyte. Telstra uses a similar billing structure.

"Prepaid users are going to chew through their credit so much faster under this system, so it's going to come as a huge shock to frequent users of Facebook," Australian Communications Consumer Action Network spokeswoman Elise Davidson said.

Caringbah mum Laura Houston, who is on a $30-a-month prepaid plan, said she didn't want to buy the extra data and SMS packages and was prepared to leave Vodafone.

"They've made changes for the better to improve their reception in recent years but I work for an online retail company and use social media all the time. I don't want to be slugged exorbitant data fees for using something small like that," she said.

Vodafone spokeswoman Karina Keisler last night conceded the changes would heavily effect 5 per cent of Vodafone's 2.7 million prepaid customers, adding it would enable the company to invest more heavily in its mobile network and was more reflective of the way customers use data.

"As much as we'd like to give data away, we need to ensure we're in a position to invest in a network to meet our customers' growing needs while offering competitively priced services," she said.

Foad Fadaghi, from analysts Telsyte, said the move was particularly odd given it went against the actions of competitors.

"It's certainly a significant positional change from Vodafone, particularly when none of their rivals are doing a similar thing," he said.

"It's definitely not going to be popular with customers."

Analysts have suggested the changes are part of an attempt by the telco to maintain its highly lucrative SMS revenues - with 37 per cent of earnings in 2011 believed to have come from SMSs.

Telcos around the world have struggled to compete with the rise of "SMS-killers" - smartphone apps that usurp the basic need to SMS - such as WhatsApp and Apple's iMessage.

In 2012, global research company Ovum forecast carriers around the world had lost $US23 billion ($21 billion) in SMS revenues - with that number likely to double by 2016.