THE government has stepped up its assault on critics of the carbon tax by accusing many Coalition MPs of privately expanding their mining shareholdings while publicly predicting doom for the industry because of the tax.

Climate Change Minister Greg Combet claimed 34 Coalition MHRs, senators or their families - about a third of the opposition parliamentary ranks - had made fresh mining investments during the past two years.

Malcolm Turnbull Credit:Glenn Hunt

Amid uproar in question time in the House of Representatives, Mr Combet named six MPs, including communications spokesman Malcolm Turnbull, as having declared fresh mining share transactions under the parliamentary register of members' interests.

''Publicly, their leader talks mining stocks down, privately, they snap up the investments,'' Mr Combet said.