NEW YORK (AP) _ Computer software specialists wasted no time Wednesday cashing in on the new federal tax code.

Even as President Reagan was signing the tax bill into law, a California software publisher began shipping a program that helps people plan their finances in light of the bill's sweeping new provisions.

Programmers at Creighton Development Inc. in Irvine, Calif., worked through the night for weeks tinkering with the software to keep up with congressional action on taxes, president Mel Creighton said.

The first copies of the revised software were shipped Wednesday, and they should be stacked on dealer's shelves by next week, he said.

''Time just blended together. It was much more of a monster effort than I ever anticipated,'' Creighton said.

The small company's fast footwork is an example of the software industry's swiftness in responding to changes in business conditions, said Kenneth Wasch, executive director of the Software Publishers Association, a Washington-based trade group.

''You've got to be fast on your toes,'' Wasch said. ''Software is more responsive to changes than any other intellectual medium,'' he said.

The financial-planning program, Finance 10, has 10 pieces, but the new tax law mainly affected the three concerning depreciation, individual retirement accounts and purchase-vs.-lease decisions, Creighton said.

Wasch agreed that Creighton Development was probably the first company out with a piece of software incorporating the new tax code, although he said other companies have similar planners in the works.