As commuters fumbled with myki cards, which became the sole ticketing system on Saturday, the Transport Ticketing Authority said the hundreds of thousands of dollars it reaped in interest would be used to ''offset the cost of public transport systems''.

About $27 million of myki money is stored on the smartcards, compared with $12 million nine months ago. Last financial year, the authority retrieved $404,000 in interest from money stored on commuters' cards, up from $150,000 the previous year. In the past five months, as commuters have flocked to the new stand-alone ticket system, the authority has reaped $306,000 in interest.

A spokesman said the interest was returned to the project budget and reduced ''the need to call upon the general pool of taxpayer funds''.

Myki money is held in a transport authority bank account, which earned an interest rate of about 3.25 per cent over the past six months.

When a passenger touches on with myki money, the cash is processed and distributed to transport operators and the Department of Transport.

Myki pass top-ups are immediately split between the department and operators.

Public Transport Users Association president Tony Morton said the interest money really belonged to commuters. ''It doesn't seem fair.''