Arvydas Nekrošius, the commission's head, asked the Seimas to extend the deadline, saying that the LRT management had been late to provide the requested documents.
Sixty-four lawmakers voted in favor of extending the deadline, 14 were against and 18 abstained.
At the initiative of the ruling Lithuanian Farmers and Greens Union (LVŽS), the Seimas established the ad hoc commission in late December, setting June 1, 2018 as the deadline.
The initiators say that the commission is looking at whether the public broadcaster rationally spends budget money, but critics describe the probe as an effort to put political pressure on the media.
Among other things, the commission is to look at whether the public broadcaster purchases products from production companies at market prices and to analyze its management structure.
A group of opposition lawmakers have asked the Constitutional Court to look at whether the establishment of the commission is in line with the constitutional principle of the freedom of expression.
LRT currently runs three television channels and three radio channels, as well as a news website. State allocations to the public broadcaster this year total 36.5 million euros, making up the bulk of its budget.