Specsavers, OPSM, Bupa Optical and others are accused of prioritising glasses sales above patient care, leaving serious health conditions potentially undetected.
A Health Services Union-led study, released on Thursday, suggests the median consultation time with an optometrist has fallen from 45 minutes to 30 minutes since 2006.
Optometrists are often the first line of detection for life-altering conditions including glaucoma, diabetic retinopathy and macular degeneration, as well as tumours and impending strokes.
Optometrist and HSU delegate Rory Dowdall said the changes put patient health at risk.
"As optometrists, a lot of people think it's just glasses," Mr Dowdall told AAP.
"But there's just so much extra stuff we can do.
"I was managing a guy who we picked up a brain tumour in his first-ever eye exam."
Optometrist and academic Matt Trinh accused some of the larger chains of prioritising bookings based on potential sales rather than health risk.
"They might turn away patients who come in complaining of a sore eye or an eye infection or something because they're not going to spend much money," he told AAP.
"They want to reserve that spot for someone who's actually going to come in and buy glasses."
The approach also could mean referring someone to an eye specialist when a condition could be easily managed by an optometrist.
Larger eye care chains impose key performance indicators on optometrists, mandating they sell a certain number of glasses or diagnostic scans.
"Usually we do stuff if it's indicated, not because you know someone tells us to do it," Mr Trinh said.
Mr Dowdall described it as a "bastardisation" of the profession, with many optometrists burned out to the point of complete disillusionment.
"A lot of my colleagues don't have the freedom to practise the way they want, and their patients are the ones who are suffering for it," he said.
The HSU is disputing in the Fair Work Commission claims by some larger retailers that optometrists fall under the retail rather than health industry award.
Union national senior assistant secretary Kate Marshall said optometrists had always been health professionals.
"What's changed is that corporate employers are running their businesses as though they are not," she said.
Specsavers and OPSM have been contacted for comment.
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