Less than three months before the Gillard government starts to collect the carbon tax on July 1, the measure continues to generate criticism from various sectors of Australian society. A tax expert questioned the carbon tax's constitutionality on Tuesday and charities warned on Wednesday of its impact on the services it provides poor people.

Bryan Pape, a University of New England professor and a tax expert said in a legal advice to the think tank, Institute of Public Affairs (IPA), that the carbon tax could be challenged on several grounds. He contends that the Commonwealth cannot tax state property, which includes carbon dioxide emissions.

IPA sought Mr Pape's legal opinion to have evidence that would help Australian states seek a legal challenge against the carbon tax. IPA Climate Change Policy Director Tim Wilson conceded that while only the High Court could decide the constitutionality of the carbon tax, there are clear grounds to a legal challenge which three of the largest Australian states would likely mount.

Mr Wilson said that while IPA would withhold the full text of Mr Pape's legal opinion, he had provided a copy of it to the premiers and attorneys-general of New South Wales, Queensland and Western Australia which have the best legal standing for a potential High Court challenge.

The premiers are scheduled to arrive in Canberra this week for the Council of Australian Governments meeting on Friday which would be the first gathering in 4 ½ years since they were in office. Labor premiers are outnumbered in the meeting by Conservative premiers, including those from the three states.

However, the activist group GetUp dismissed Mr Pape's opinion as part of a meaningless stunt by IPA. GetUp National Director Simon Sheikh challenged the IPA to release in full Mr Pape's legal advice so it could be scrutinised.

In 2009, Mr Pape failed in his challenge to the federal government's $900 stimulus cheques given to 8.7 million Australian taxpayers.

Queensland miner and billionaire Clive Palmer vowed in mid-March to initiate a High Court challenge against the carbon tax, while Opposition leader Tony Abbott had promised to axe the tax if he becomes prime minister. Queensland Premier Campbell Newman said on Tuesday that if he comes across a credible legal opinion against the unpopular measure, he would pursue a legal challenge to it.

Meanwhile, leading charity groups warned of the negative impact of the carbon tax on the services they provide to poor Australians.

The Salvation Army said it fears people would just dump used furniture and clothing they no longer use outside charity's stories instead of pay rubbish tip fees caused by the carbon tax. The group estimated it may increase the annual landfill bill of charities by $3.5 million.

That, in turn could reduce the social services it offers to about 300,000 people yearly, including emergency shelter and drug and alcohol counseling.

The Herald Sun reports that one fourth of good collected by the Salvation Army, St Vincent de Paul and other Australian charities are just dumped in front of their offices. The Salvation Army estimates it would cost them between $687,000 and $1.25 million in extra landfill fees when the carbon tax is introduced by July 1.

However, Climate Change Minister Greg Combet assured the charities that the government's Community Energy Efficiency Programme would be tapped to lower the energy usage of community organisations.