The federal government is looking to claw back $4 billion in overpayments.
Mr Porter characterised the letters as polite.
He declined to provide the number of people who are challenging the debt letters.
Mr Porter said the letter asks people to provide more information which they can do online.
He dismissed claims there were problems with the Centrelink website.
"If anyone is having difficulties they can seek an extension of time if they wish," he said.
Mr Porter said the high volume system was "working incredibly well".
Welfare recipients are tweeting criticism of the system under the hashtag #notmydebt.
Labor's spokeswoman on human services Linda Burney has called for the system to be suspended.
"Get it right before threatening people - not that hard," Ms Burney tweeted.
She said no MPs who checked the emails to their electorate office could possibly say the system was working.