The Turnbull government is to consider a ban on the $100 note and a crackdown on all but small cash payments as part of an assault on the cash economy to be unveiled in Monday's mid-year budget update.

"There's nothing wrong with cash, the issue is when people don't declare it," said Financial Services Minister Kelly O'Dwyer ahead of the announcement.

"The cash economy is worth 1.5 per cent of GDP, that's about $21 billion. If we can get a percentage of that, it's revenue owed to the Australian people."

The amount of money at stake was more than the amount missing as a result of corporate tax avoidance.