The charity Credit Action said the nationalised lender was more than twice as likely to repossess homes as other lenders, and was not flexible enough in dealing with borrowers who defaulted on their payments.
More than 19,000 homes were repossessed in the first half of this year - around 4,000 by Northern Rock.
Earlier this week the bank said it was "well ahead" of its Government loan repayment target, having paid back more than half the £26 billion owed, to leave £11.4 billion outstanding as at September 30.
Chris Tapp, the charity's director, said the Rock's eagerness to repay taxpayers' money meant it was treating its struggling customers harshly.
"As soon as people fall behind, they have moved to start repossession proceedings," he said.
"We are not talking about people who are trying to avoid paying, but people who are struggling in the short term.
"There was a case a couple of days ago where because it was the end of the summer holidays, a family had to buy a load of new school clothes, and as a result fell behind with their payments.
"Northern Rock went straight to begin repossession proceedings."
Q&A: What to do if you're struggling to pay your mortgage
Before running into funding problems last summer, Northern Rock was one of the UK's biggest and most aggressive mortgage providers, advancing loans worth as much as 125% of home values.
But it was forced to turn to the Bank of England for emergency support after the money markets froze, leaving the group facing a funding crisis.
Mr Tapp said the bank's "irresponsible" lending policies in the past explained why it had more borrowers who were likely to default, but did not excuse the "aggressive" approach it was taking with them.
More use could be made of mortgage holidays and the like to help homeowners struggling to make payments, he said.
"They are being quite aggressive in terms of their use of the courts in going for repossession, but for a lot of people they don't need to get to that point, if only Northern Rock would be more flexible with them in the first place," he said.
The Rock's publicly-owned status was part of the reason for its approach, he added.
"Because they are nationalised they feel it necessary to get back taxpayers' money as quickly as possible," he said.
"The Treasury is in a position to lean on them a bit to take a more flexible approach - they do have the controlling stake in the bank.
"We have seen the Treasury is ready on executive pay to take quite radical action over what they will force banks to do, and we would like to see similar action on Northern Rock."
Northern Rock's nationalisation in February led to 1,500 job losses as it scaled back activity to pay back the Government.
The lender has been reducing the size of its mortgage book in order to pay back its Government borrowing, and repaid £15.4 billion during the nine months to September 30.
But the group's mortgage arrears figure jumped by nearly 60% in the past three months, reflecting the fact that it has been left with poorer quality loans.
The number of houses in Northern Rock's possession jumped 491 to 4,201 in the three months to September 30.
Most of the repossessions were for properties secured with a "Together" mortgage, which allowed buyers to borrow up to 125 per cent of the property's value.
The bank's chairman, Ron Sandler, said: "I refute any suggestion that our position has been anything other than commercial and consistent with the way this bank has operated in the past.
"I would deny strenuously that we have been overly aggressive (in terms of repossessions)."
In August, Northern Rock said it was increasing its debt management staff from 185 to 500 to cope with growing numbers of mortgage defaulters and those having difficulties meeting payments and remortgaging.
A spokeswoman for the Council of Mortgage Lenders (CML) said Financial Services Authority regulations required banks and building societies to use repossession only as a last resort.
"Lenders have to work with borrowers to explore all possible repayment options before they move to repossess," she said.
The CML predicted that 45,000 properties would be repossessed this year - up from 26,200 last year, and at a level last seen in the mid-1990s.
Repossessions peaked in 1991, when 75,500 properties were reclaimed, the CML said.
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