The court is expected to rule on Friday whether the Rudd government exceeded constitutional powers in legislating for the payments - up to $900 each for eligible taxpayers - due to distributed from next Monday.

George Williams, a senior law lecturer at the University of New South Wales, says taxpayers will get their money "one way or another" because the government would use another vehicle to implement the measure.

"I don't think people should be worried," he told ABC Radio.

But they might not be receiving the cash bonus as quickly as next week if the High Court rules the payments invalid.

"It's possible it would be delayed, maybe even quite significantly."

If the court rules against the payments there would be a "massive shift of power and influence" away from the commonwealth to the states, Professor Williams said.

The academic who initiated the challenge agrees with Professor Williams that the tax bonus will be paid somehow.

"Tomorrow, if I'm successful the government could still give the bonus just through ... giving a rebate and letting the money flow through the PAYG system," Bryan Pape told ABC Radio.