Senate Finance Committee Chairman Bob Packwood (R-Ore.) has warned opponents of a proposed public employe pension tax change to be on guard, despite a big victory last week for the no-tax forces.

The committee surprised and delighted many feds Wednesday by rejecting a House-passed plan to slap an immediate tax on pensions of civil servants, state and local government employes and private sector workers who pay into their own pension plans. They now enjoy a tax- free period after retirement.

Under current law, such employes -- there are 17 million of them -- do not pay pension taxes until they have recovered all their contributions. That money has already been taxed as part of salary. The so-called recovery period lasts about 18 months for the typical government retiree, and it is important for financial planning purposes.

The House tax reform bill would end the recovery period. Persons retiring in the future would have to start paying taxes immediately on a prorated portion of their pensions. White House officials estimate that the change in tax laws would raise $6 billion to $8 billion in the next four years.

The Senate Finance Committee is working on its own tax reform bill.

But despite the vote last week, federal and postal organizations opposed to the pension tax change are worried. They say proposals to change the system would be unfair for midcareer and older employes, would disrupt their retirement planning and could cause many of the 200,000 retirement-age feds to quit early to beat any tax change deadline.

"Packwood was remarkably candid," said George Gould, chief lobbyist for the National Association of Letter Carriers. "He said there is no agreement that the pension tax change won't come up again." Gould said that Packwood, who supports the pension tax change, "was very fair and warned us to stay on the alert."

Gould, one of the best-connected lobbyists on Capitol Hill (and president of the Democratic Club), said the committee tax bill has a revenue shortfall of $25 billion to $30 billion. And, given the choice between raising taxes and seeking new revenue sources, Congress will probably go for the latter.

"What we are afraid of," Gould said, "is that people will look at the victory last week and relax. Then we could get clobbered later on in the committee, in the Senate," or when the tax bill goes to a Senate-House conference, probably this fall. Out, but Not Down

Although he is no longer public relations director for the American Federation of Government Employees, George Ryder hasn't given up government watching. He continues to write a column for AFGE's magazine. His lead this month: "Now that I have retired, I've restricted my role in running the government to maintaining my registration as a Democrat, praying I live until 1988 and gritting my teeth during Reagan speeches . . . . " Meetings

The National Association of Government Communicators capital area chapter will have its April 29 luncheon at the Touchdown Club. Department of Education's James Bencivenga is the speaker. For reservations, call Beck Keiper tomorrow at 566-1794.

The League of Federal Recreation Associations' April 25-27 conference will be at the Hershey (Pa.) Convention Center. Call Mary McKey at 554-6910.

The Bolling-Andrews Air Force Base Alumni Association is having a dinner meeting May 3 at the Andrews NCO club. For reservations or information, call James Perkins at 584-3427.