Graduates who fall foul of the Student Loans Company face paying back thousands of pounds more after it emerged the firm has the right to penalise students with extra interest for simply taking a three-month trip.

Guardian Money has been contacted by one Oxford graduate who says she has faced a nightmare dealing with the SLC after it raised the interest on her £42,000 loan from 0.9% to 3.9% for failing to keep to its rules. She is just one of 33,000 graduates caught up in the SLC’s controversial non-compliance penalties, which can be triggered by a person leaving the UK for 12 weeks without telling the firm.

Jenny Richards’ apparent crime was to go to Australia nine months after she graduated in 2015, to take up a research assistant post at the University of Adelaide. Richards, from Burwell in Cambridgeshire, took on largely voluntary work, combined with paid employment six hours a week, to help cover her costs. She earned far less than the amount that would have required her to make loan repayments.

The SLC is so opaque that an Oxford graduate, my parents and my MP have been unable to get a satisfactory response

“The letter from the SLC stated that if you were working abroad for more than three months you had to fill in the overseas section,” says Richards. “I gave it details of my income, the savings I was living off and contact details for Australia. As I had heard that the SLC could be difficult, I also gave it details of my return date, my plans for further study and future UK contact details.”

She says her mother who was visiting her took the forms and supporting evidence back with her to post them from the UK. One page was sent later than the others, but all were posted before the April 2016 deadline, Richards says.

“A month later I received a letter saying that due to my ‘failure to respond’ to its letter a ‘non-compliance’ interest rate of RPI plus 3% was being applied to my account because I hadn’t given the information it needed – but I had.”

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For more than 18 months, Richards, her parents and even her MP have been trying to get the SLC to confirm that her loan is not growing at the higher rate, but to no avail. “Despite countless letters and phone calls I have never received confirmation that the interest rate has been removed and that the additional debt accrued has been removed from my total debt,” Richards says. “What scares me the most in this whole process is that the SLC is so opaque that an Oxford graduate, my parents – both with Cambridge degrees – and my MP, Lucy Frazer, have been unable to get a satisfactory response from a company that thousands of young people are massively in debt to.”

A spokesman for the SLC told Money that Richards’ penalty charge had been applied as a result of an administration error, which was reversed a few weeks later. “We apologise for any inconvenience caused and will be sending her a letter to confirm that the interest rate was reversed in July 2016. She has not been financially disadvantaged as a result of this.”

He confirmed that anyone with a student loan must tell the firm if they go abroad for three months or more – whether they are earning or not. They are also required to show how they are funding the trip. Failure to do so counts as non-compliance, at which point the SLC will apply the penalty charge until the student provides the information. These fines were introduced in April 2016.