Australia's three biggest mobile contractors have slashed the amount of data offered in their plans, with some cutting their offerings by up to 40 per cent on what they provided last year.

"This is really a sleight-of-hand price increase," said Elise Davidson, a spokeswoman for the Australian Communications Consumer Action Network, the peak body that represents consumers on communications issues. "In most cases, plans are staying at the same price point but you get less included, especially in terms of data."

Unkindest cut ... your plan offers less data.

The worst offender is Optus. All of its plans now offer less data and credit, and this is more pronounced on higher value offerings. Customers on the $80 cap get 2GB of data and $850 of credit, which is half the data and $50 less than was being offered in last year's $79 cap.

The value disparity on Optus's $99 Timeless consumer plan is wider, with only 3GB of data included compared to 5GB last year.