Senate Finance Committee Chairman Bob Packwood (R-Ore.) said yesterday he did not author a modified flat-tax system that was laid out to his committee Thursday, and he repeatedly attributed responsibility for the proposal to the relatively unknown chief of staff of the Joint Committee on Taxation.

Adding another element of confusion to the sputtering tax-overhaul process, Packwood declined at a news conference to state his position on the plan, which would abolish all itemized deductions for individuals while cutting overall taxes through lower rates. But he did say he favors the plan's proposed top individual tax rate of 25 percent.

The author of the plan, Packwood said several times, was chief of staff David H. Brockway, whom Packwood repeatedly beckoned before the microphones to answer questions about details of the package.

Asked to whom the proposal belonged, Packwood said, "These are options Dave brought us. All I wanted was to get the committee back to what we have to do to get to a top rate of 25 percent." The package also includes a tax rate of 15 percent for lower income levels.

Packwood's answer left unclear precisely what tax package is pending before the Finance Committee or exactly what the committee will work on in coming weeks.

Last week, Packwood suspended votes on his original proposal after numerous amendments had drained it of $29 billion in projected revenue over five years. And he announced that the committee would meet in private to go back to "square one" on tax overhaul.

It was at the first of those meetings Thursday that Packwood aired the modified flat tax, the sharpest departure from the current tax code of all the proposals, including the House-passed bill, President Reagan's plan and the first Packwood plan.

Reaction was mixed as senators questioned the political feasibility of eliminating such deductions as the one for home mortgage interest but welcomed a turn toward the basics of tax revision. The panel is scheduled to meet in private again Tuesday, when modifications to the new plan are to be discussed.

Until then, at least, Packwood said, his earlier plan is "put aside in abeyance." Yet Packwood and his aides said they do not know whether the new plan will become the starting point for writing a new tax bill, and he pointed out that it is still in rough form.

"The process is one designed to focus on the basic policy decisions that have to be made if you want to do fundamental tax reform," Brockway said. "It is not a specific proposal as such."

Asked whether he can accomplish his previously stated goal of getting a bill out of committee by early May, Packwood said, "I just take it day by day. I don't have any set timetables . . . . The longer I'm at this the less I'm able to predict." Aides said later that Packwood hopes to have a final bill to Reagan by Aug. 15, as he has predicted in the past.

The marching orders Packwood gave to the staff of the nonpartisan Joint Committee earlier in the week were essentially to design a simple tax system with a top rate of 25 percent now 50 percent , a personal exemption of $2,000 now $1,080 and a smaller tax increase for business than the $100 billion-plus in the major proposals. But he said the plan must collect the same amount of revenue as the current tax code.

The plan would raise business taxes by $70 billion over five years, while reducing individual taxes by $95 billion and raising excise taxes by $25 billion. This represents a smaller average cut for individuals -- 4.7 percent -- than is proposed in any of the other tax plans. Packwood said he is disturbed that the Brockway proposal would provide a relatively small tax cut for some middle-income taxpayers -- as little as 1.3 percent for those earning $30,000 to $40,000 per year.