Australia's missing $100 bills could be being stockpiled by criminals or those seeking to avoid taxes, an expert says, as momentum to axe high-denomination bills builds worldwide.

The Reserve Bank's website says there are 300 million $100 notes in circulation, compared with 165 million $5 notes, 116 million $10 notes and 157 million $20 notes.

The only note more abundant is the $50, with 608 million currently in use.

However, the $100 bill is rarely seen.

Swinburne University adjunct professor in marketing Steve Worthington said speculation was rife $100 bills, which are rarely stocked in ATMs, were being used for tax evasion and crime.

"The RBA is printing a lot more $100 notes than are out there, so one must speculate they're either being hoarded by people to avoid paying tax or by criminals to store wealth," he said.

While the Royal Australian Mint recently flagged the 5 cent coin as being on the chopping block, Professor Worthington said the idea of axing the $100 bill had been mooted, with proposals already on the table in the United States and Europe to eliminate the $US100 bill and the 500-euro note.

"There is suggestion this kind of high-denomination note is used to fund terrorist activity," he said.

"It's not just [ordinary] criminals, but terrorist groups are also using cash."

A Harvard University study recently recommended doing away with the most popular banknotes used to evade the banking system: The $US100, the 500-euro note, the 1,000-Swiss-franc note and Britain's 50-pound note.

The study said some two thirds of the total $US1 trillion in $US100 notes in circulation were outside the US.

However, researchers said the 500-euro note was gaining popularity for its size and portability and had become so prized in underground finance, it could trade at more than its face value and was known in some circles as a "Bin Laden".

"Such notes are the preferred payment mechanism of those pursuing illicit activities, given the anonymity and lack of transaction record they offer, and the relative ease with which they can be transported and moved," researchers said.

The study, led by former Standard Chartered bank chief Peter Sands, suggested removing high-value bills from circulation would have little impact on legitimate business, as the rise in electronic payments had rendered them "an anachronism".

But eliminating them "would make life harder for those pursuing tax evasion, financial crime, terrorist finance and corruption" and "disrupt their business models".

European Central Bank president Mario Draghi said on Monday the ECB was considering eliminating the 500-euro note, while former US Treasury secretary Larry Summers has called for central banks to begin a moratorium on printing new high-value notes.

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Less cash, not cashless, society

Despite the popularity of wireless or card-based payment methods, Professor Worthington said he believed Australia was moving towards a "less cash" rather than a "cashless" society.

"Whatever everyone says of the wonderful options in terms of contactless cash and tap and go, there's still a psychological attachment we have to having some cash on hand," he said.

He said recent incidents, like Telstra's mass network outage last week, had highlighted the need for cash.

"I couldn't use my phone to do anything, let alone pay with it," Professor Worthington said.

"Sometimes a bank's IT system crashes, you can't use a card, you can't get money out.

"We keep it in case of emergencies when you can't use [your cards]."

He said some people simply preferred to use cash, and said it was possible some of the missing $100 notes were being stockpiled by ordinary Australians.

"That is very possible, particularly in the current times where we have economic uncertainty," he said.

"No doubt there are some people who prefer to keep some of their wealth in cash ... I think it's a very good point that there are probably people who have an element of cash that they think they'd like to have in case things come to a point where they need it.

"Higher denomination notes are easier to hide and hold.

"We like cash because it's untraceable, it gives us anonymity and it's acceptable everywhere."