A new O'ahu surcharge on the state excise tax brought in only $14.2 million for the city's mass-transit project during the first quarter of the year, the Department of Taxation announced.

But a state tax official warned not to read too much into that number, noting that it does not accurately reflect a full, three months' take for the surcharge.

It's been estimated that the additional 0.5 percent surcharge on top of the 4 percent general excise tax would net between $150 million and $164 million in its first year, and $3 billion over the 15-year life of the tax. That would come out to between $12.5 million to $13.6 million each month.

While the $14.2 million collected represents O'ahu surcharge taxes processed as of March 31, about $12 million of that came from the month of January, Titin Liem, special assistant to state Tax Director Kurt Kawafuchi, said last week.

February general excise tax returns were not due until April 2 because March ended on a weekend, so a majority of February's surcharge taxes were received or processed after April 2, Liem said. March returns aren't due until April 30.

The first-quarter numbers also do not reflect the 7 percent of businesses who choose to file their excise tax returns on a quarterly or semi-annual basis, which are due on April 30 and July 30, respectively, Liem said.

Additionally, she said, an unusually high 15 percent of the returns received through March erroneously left blank the second page, which includes the section where taxpayers report their county surcharge tax.

The second-quarter report with collections through June 30 should include three months of returns (February, March and April) but "while it's a better picture, it's still not the whole picture because we know that the semi-annual filers don't have to file until the end of July," Liem said.

Reach Gordon Y.K. Pang at gpang@honoluluadvertiser.com.