March 10, 2014 5:14 p.m

SÃO PAULO—The Brazilian Central Bank still hasn’t decided how many of the forex swap contracts that expire on Apr. 1 it will offer to roll over, and traders and investors who think the bank has decided not to renew all of them could be wrong, the bank’s director of monetary policy said Monday.

“The central bank could renew all of them, or not; we have that choice,” said Aldo Mendes, the central bank’s monetary policy director, in a telephone interview with The Wall Street Journal.

The swap contracts are an instrument the central bank uses to help reduce volatility in the foreign exchange market by providing investors with a predictable way to protect themselves from big changes in the value of the Brazilian real. The bank uses the rollover auctions to give companies the chance to renew expiring swap contracts.

The bank announced on Friday its first rollover auction, of up to $500 million, to start tackling the $10.75 billion worth of contracts expiring Apr. 1. With 16 work days from Monday to the last day of March, that would add up to renewals of only $8 billion if all the remaining rollover auctions were the same size as Monday’s.

The central bank could nevertheless still decide to change the auction maximum and roll over all of the expiring contracts, Mr. Mendes said.

“The fact that we did less today doesn’t mean we’re not going to” decide to roll them all over, he said.

The bank started the program of regular swap contract auctions last August, when the real reached a five-year low against the dollar. Before that, the bank had announced auctions randomly.

When the bank announced the program of regular auctions, the real had fallen more against the dollar than its emerging-market peers, hitting its lowest point since 2008. In the days before the bank announced the regular program it had been forced to hold as many as three swap auctions in one day.

The auctions have achieved the bank’s goal of providing markets with predictability regarding the availability of dollars and reducing volatility, Mr. Mendes said.

The bank is “still very happy, absolutely, with the results,” he said. “Our goal has been reached, and our main goal with the swaps is to give the economy the opportunity to hedge exchange risk.”

The original program consisted of four auctions of swap contracts each week, on Monday through Thursday, of up to $500 million each, and a dollar repurchase contract auction on Fridays of as much as $1 billion. The bank said at the time that the program would last through the end of 2013.

In mid-December the bank said it would extend the program, with some changes, through at least the end of June 2014. The bank abandoned the dollar repurchase auction on Fridays and added another swap contract auction in its place. At the same time, the bank cut the upper limit for the auctions, to $200 million a day.

The decision to extend the program into 2014 was based on demand from the market, albeit at a lower level than the bank had been offering in 2013, said Mr. Mendes, and no decision has been made yet on whether the program will be extended again past June.

The U.S. Federal Reserve Bank announced on Dec. 18 of last year that it would start cutting its economic stimulus spending the following month, to $75 billion a month from $85 billion a month previously. The bank has since announced another cut, reducing buying to $65 billion a month.

World financial markets had been expecting such an announcement. Concern about the effects of less spending by the Fed was an important factor in the decision to extend the swap auction program through June, Mr. Mendes said.

The central bank considers the effects of the so-called tapering of the Fed’s spending when it makes decisions, he said, adding that markets have already reacted to the start of the reductions.

“It’s important to point out that the biggest impact of the impact of tapering was already priced in last year,” Mr. Mendes said.