Professor Anne Twomey told a Senate inquiry into the controversial $100 million scheme the government did not have the power to award grants under the constitution.

She said the government had either deliberately or uncaringly breached the constitution, setting a bad example for Australians.

"It breeds contempt and disengagement from politics," she told senators in Melbourne on Thursday.

"I see it in my classes all the time with my students - it's dangerous and it's being fuelled by the Commonwealth government's disregard for the constitution and fundamental constitutional principles such as the rule of law."

She said people prosecuted for receiving minuscule amounts of government money above their welfare entitlements were incredulous the government could get away with spending public funds without authority.

"On its face, the minister had no power to make the grants in question," Prof Twomey said.

Nationals senator Matt Canavan raised the prospect of the government being given authority by passing the budget appropriations bill.

But Prof Twomey shot down the idea.

She also said money for sporting clubs should be handed out through state and local governments.

Prof Twomey questioned if any senators could seriously say the framers of the constitution envisaged sporting clubs getting federal money.

"Frankly it's madness and no sane person would devise such a system. Certainly those who wrote the constitution were sane and did not devise such a system."

She took aim at the government's defence of the program, saying there was no point in arguing the minister had authority under the program's guidelines.

The constitutional expert said the government had so far failed to outline where its legal power to award grants to sporting clubs came from.

"The onus is on the Commonwealth to identify it and its silence in that regard has been deafening," she said.

"If there is an adequate legal explanation, then the government should just give it and people like me would stop complaining."

Prof Twomey said it was of great concern if ministers believed they had unlimited discretion over public funds.

"They cannot rely on ignorance of the law for their exculpation," she said.

She was bemused by the focus on a potential breaking with caretaker conventions through changing the successful projects after the election was called.

"It's much worse to breach the law and the constitution," she said.

Australian Associated Press