By Josie Cox 

The Australian dollar strengthened sharply against the U.S. dollar on Tuesday, just seconds before Australia's central bank surprised some investors by holding rates steady.

The action in currency markets just ahead of the announcement prompted head scratching among some market watchers. In a note, traders at Citigroup said the move, which correctly anticipated the direction of the Australian dollar, raised "a few questions about the possibility of a data leak."

A representative with the Reserve Bank of Australia wasn't immediately available for comment. A spokeswoman for the Australian Securities and Investment Commission said: "We cannot comment specifically, however as you may expect we would review any unusual trading as part of our routine monitoring and surveillance activities."

In May of 2014, an employee of Australia's government statistics office and an employee at one of the country's biggest banks were arrested in a case alleging insider trading that used leaked government data.

The data is held under a strict embargo and released to financial markets at the same time each month. Authorities allege in the 2014 case that the bank employee used the leaked numbers to trade foreign-exchange derivatives--generating a profit of about 7 million Australian dollars (US$6.6 million).

Many investors and traders had expected the RBA to cut its benchmark interest rate from a record low 2.25%. Falling rates tend to make a currency relatively less attractive to hold. The central bank kept rates unchanged, citing concerns about frothy housing prices.

The absence of a rate cut forced traders to cover bearish bets, causing the Australian dollar, also known as the Aussie, to rally.

The Aussie rose from around US$0.777 to around $0.782 in the minute before the official announcement. It later pared gains and recently traded at US$0.78.

Write to Josie Cox at josie.cox@wsj.com

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