SuperGold Card travel costs are soaring as more people pick up the free scheme, and the Ministry of Transport says there is now a $4 million funding shortfall.
The card, which is available to pensioners and veterans, entitles users to a range of Government concessions, including free public transport during off-peak periods - 9am-3pm on weekdays.
Ministry chief executive Martin Matthews this morning told a parliamentary transport committee that the initial funding for the travel scheme was set at $18m in 2008.
"The level of patronage has grown significantly to where the actual cost of the scheme is somewhere around $22 million," he said.
Extra funding was provided two years ago, but Mr Matthews said consideration now needed to be given to a long-term solution to fill the funding gap.
Speaking outside the committee, Mr Matthews said he could not say whether the number of baby boomers entering the scheme was the reason for the current pressure, but it was something that needed to be taken into account for future.
"If you take a really long view, we're going to have more people over 65 and so if they make use of the card then the cost of the scheme will continue to grow,"he said.
Mr Matthews said the issue had to be resolved this year, and the ministry would be speaking to the Government over the next few months.