Cleaning toilets is a fair way to punish students, a Cambridge dean has insisted, after it was revealed university offenders were also made to "dig trenches" instead of paying fines.

The head of St John's College has said manual labour is a fairer form of punishment as some students cannot afford to repeatedly pay fines.

"The dean's policy is to make such an order in preference to imposing a fine," a statement from the college read. "The college recognises that the effect of a fine varies according to students' ability to pay it."

Last week The Cambridge Student revealed one student at St John's was made to dig and fill in an irrigation trench, while another had to clean toilets for an hour every day over six days.

The student paper also claimed fines at other colleges were being used to fund staff coach trips. Five students were fined £1,205 for having a party, while others were reportedly penalised for setting fire alarms off for boiling kettles and leaving their rooms untidy.

Since October last year, more than £38,000 was collected in college fines, with Newnham College collecting a quarter of the total amount.

Pete Mercer, the National Union of Students' vice president, told the Guardian universities are treating students as "cash cows".

"It's vital that universities considering levying fines take student hardship into account. That institutions are racking up such huge sums in fines suggests that they are gratuitous, and are treating students as cash cows.

"If money is collected through fines, it must be put back into helping students through things like hardship funds and library resources."