The Nigerian National Petroleum Corporation (NNPC) said half a billion litres were taken from the pipeline that runs north-east from Lagos.
Long queues have formed at petrol stations across Nigeria in recent days.
Governments blame pipeline vandalism and theft in the oil sector for fuel shortages and damaging the economy.
An NNPC subsidiary told a Senate committee that "incessant hacking" of the System 2B pipeline had "made the task of providing seamless flow of petroleum products to retail outlets more burdensome".
The pipeline stretches 250km from the financial hub Lagos to the city of Ilorin.
The company said it was working to resolve the issue.
"We have been pushing 35 million litres every day to the market and there's no reason why there shouldn't be fuel," said Esther Nnamdi Ogbue, managing director of the Pipelines and Product Marketing Company, a subsidiary.
She blamed "sharp practices" such as hoarding in some areas.
Nigeria is Africa's biggest oil exporter, but a lack of refining capacity means drivers rely on imported petrol and there are frequent fuel shortages.
In May the country was brought to a virtual standstill after importers shut depots over subsidy payments.
President Muhammadu Buhari has been urged to scrap the subsidy scheme, which critics say is rife with corruption, but a previous attempt to stop the payments led to violent mass protests in 2012.
However, in August NNPC managing director Emmanuel Kachikwu said the subsidies were an unsustainable drain on the economy, which has suffered as global oil prices have fallen.
Thieves have stolen nearly $250m (Â£165m) of oil from a single pipeline this year, Nigeria's state oil company says.