New orders for durable goods turned up in August by a modest 0.8 percent following much larger drops in June and July, the Commerce Department reported yesterday.

Separately, in figures provided privately to administration officials, the department estimated that after adjustment for inflation, the nation's output of goods and services grew at a seasonally adjusted annual rate of more than one percent during the third quarter.

Real gross national product fell at a 2.3 percent rate in the previous three months.

But government analysts were quick to say that the mild rebound in the July-September quarter does not mean that the economy is not in a recession because they expect a renewed decline, perhaps a sharp one, in the fourth quarter.

The rebound is the result of a pickup in consumer spending, continued strength in housing construction, an increase in net exports, and continuation of inventory accumulation by business, analysts said.

A number of private forecasters are predicting businessmen will eventually have to reduce their stocks to get them back in line with sales, and that when that happens, real output will drop rapidly.

Part of the increase in consumer buying, particularly for automobiles, likely represents "borrowing" sales from later months, analysts said. Some of the buyers attracted to 1979 model cars by large cash rebates, for example, probably would otherwise have bought 1980 models.

The estimate of third quarter GNP probably will bolster the administration's refusal to consider a tax cut to stimulate the economy at this time, as several members of Congress are urging. Several officials, including Treasury Secretary G. William Miller, have said it is "premature" to propose a tax cut.

Since reaching a high of $83.1 billion last March, new orders for durable goods have dropped an average of 2.4 percent per month, the Commerce Department said. The amount of orders placed last month was $73 billion.

A significant increase in orders for machinery reported for August, up $1.3 billion to $23.3 billion, still left that category of orders rising by less than 0.6 percent a month this year compared to a 1.4 percent monthly rate during 1978, the department said.

New orders for non-defense capital goods, primarily equipment bought by business, dropped by 1.6 percent, largely as a result of a $500 million decline in the aircraft and parts sector.

The modest overall rise in orders was less than the increase in shipments from July to August. Thus, for the second month in a row, the backlog of unfilled orders declined.

Once an adjustment for inflation is made, the backlog slipped even more, analysts noted.

For durable goods manufacturers outside the transportation area, new orders rose by $1.1 billion to $57.8 billion. But with shipments running at $57.9 billion, their order backlogs were reduced.

Orders for primary metals fell $300 million, or 3 percent, to $10.6 billion, mostly because of a big drop in steel orders. With the exception of a 4.9 percent increase in June, steel orders have declined steadily from a January high of $7.3 billion to $4.5 billion in August, an average monthly decline of 5.6 percent, the department said.