The University of Melbourne science student is among thousands of young Australians who are converting their lecture and textbook notes into cash.

Hundreds of students have paid $3 to access Mr Weng's neatly formatted biology and chemistry notes, which he uploads on a website called NoteXchange.

"It's a little bit of pocket money on the side," the 19-year-old said.

Students trust his work – he achieved a perfect ATAR ranking of 99.95 and receives good marks at university.

"I'm doing this for myself anyway. I may as well spend a few more minutes putting them online," he said. "It helps students understand some weird concepts."

There has been a proliferation of note-sharing websites, where Australian students submit their VCE, International Baccalaureate and university notes and profit from their sale.

But not everyone is happy with their existence.

University of South Australia academic integrity expert Tracey Bretag said buying notes led students down a "slippery slope" to purchasing essays.

"These sites are dangerous, even if they are legitimately trying to just provide notes," she said.

"These notes are available freely to students at their own university. Go to class people!"

She said that while many students thought buying notes gave them academic edge, it actually disadvantaged them.

"They cheat students of an opportunity to learn. When you get into an exam, those notes are not going to be that useful. It can undermine your learning."

Associate Professor Bretag is conducting federally-funded research into contract cheating and assessment design, which will also examine the proliferation of note-selling websites.

But Richard Hordern-Gibbings, the co-founder of Nexus Notes, said it was unfair to compare note-selling websites with those that sold essays.

"You can't plagiarise a set of notes," he said. "They are a learning aide."

The 27-year-old founded Nexus Notes with three of his friends while he was studying commerce at Adelaide University in 2010.

He regularly swapped lecture notes with his friends and saw a gap in the market.

"Note sharing has been going on for decades but there was no way to monetise it," he said.

"If you didn't have those existing friendships groups, you were left out. This was a way of democratising it."

The business now has thousands of customers and operates in Australia, New Zealand and has a small presence in the US. Some students make more than $500 a month by selling their notes on the site, which charges $35 for each set.

He said some students spent 60 hours compiling their notes for a subject.

"They have achieved a really good mark, and it's a chance for them to say I have done all this work, and maybe I can make some money from it."

The dangers of solely relying on online notes have played out at at least one university, where students referred to the same out-of-date information during exams.

A University of Melbourne spokesman said the university had no problem with students sharing their course notes, as long as they submitted work that was their own, and met strict academic integrity rules.

"However the university would not be supportive of notes being shared that includes copyright materials and the intellectual property of the university," he said.