The extension was made on Saturday after only about one in eight of those eligible to vote signed up during the initial phase.
By Friday, the fifth day of registration, 131,635 of an estimated one million eligible voters in 14 countries had registered, the Geneva-based International Organisation for Migration's Iraq's Out of Country Programme said.
"We are extending our operation in an effort to provide Iraqi voters with as much access to our centres as possible," IOM head Peter Erben said in a statement.
"We are hopeful that we will see a significant increase over this weekend that many more Iraqis will come forward to participate in this historic opportunity," he told the BBC's Today programme on Saturday.
Registration had been set to close on Sunday, but the IOM said it would remain open until Tuesday at the 74 registration centres signing up voters, many of whom fled into exile during
Saddam Hussein's rule.
The IOM has also eased registration rules by accepting an Iraqi passport instead of at least two documents to prove their identity, and extended operating hours at polling centres in several countries.
From Canada to Jordan, about a
million Iraqis are elgible to vote
Several Iraqis hired by the IOM to staff and run 12 registration centres in Jordan said a trickle of people showed up on Saturday despite a lavish IOM-financed advertising campaign in the local media.
"There are few people who are coming and many are suspicious of the whole process while others are afraid their registered names could be used by the (Iraqi) authorities against them at some point," an Iraqi electoral worker who identified himself as Ali Salim told Reuters.
The deadline for expatriate Iraqis to register for voting in the 30 January elections has been extended by two days due to low registration figures.
Amman-based Western diplomatic sources said registration was lagging behind the IOM moves. In Jordan, for example, only about 5000 of at least 150,000 eligible voters had registered.
The January elections will pick a 275-member parliament which will in turn select a cabinet and oversee the drafting of a permanent constitution.
Registration began on Monday in 14 countries - Australia, Britain, Canada, Denmark, France, Germany, Iran, Jordan, the Netherlands, Sweden, Syria, Turkey, the United Arab Emirates and the United States.
Countries with the largest numbers of Iraqis are: Syria, with an estimated 250,000 voters; the United States, with at least 200,000; and Britain with 150,000.
Voters can cast ballots at the same centres where they registered to vote from 28-30 January.