A Senate estimates hearing was told the scheme will cost $500 million more for the 2022/23 financial year than what the government forecast in the October budget.
At the time, the budget said the scheme would cost $35.5 billion for the financial year.
Projections have shown the scheme will cost $44.1 billion by 2024/25 and more than $50 billion the following year, overtaking annual costs for Medicare.
The scheme's acting actuary David Gifford told estimates the cost of the scheme was $300 million more than expected from July to December in 2022.
Costs exceeded projections by $200 million during January 2023 alone.
Payment predictions were set to be $2.7 billion during the month of January but the actual payment amount was $2.94 billion.
However, Mr Gifford told the hearing the January figures should not be seen as part of a growing trend.
"There is a degree of fluctuation month-to-month in payments ... processing will vary a bit," he said.
"We haven't finalised our projections for the year. We'll be doing that over the next month or two in preparation for the budget."
NDIS Minister Bill Shorten announced a review of the scheme last year, with the program set to cost an extra $8.8 billion across the next four years.
At the time, Mr Shorten said the review would examine wastefulness within the scheme, with findings to be handed down in October.
The Senate committee was also told cases being examined by a task force cracking down on fraud within the scheme were worth about $250 million.
However, One Nation senator Malcolm Roberts said the money would not go far, even if it was all reclaimed.
"Given the $52 billion (future cost of the scheme) and given the deep concerns about fraud, I'm wondering if we're trying to bail out the Titanic with a coffee mug," he said.
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