The NSW government has stopped short of mandating mask use on public transport, instead introducing a new app feature that predicts when trains will be crowded.

The NSW government is still resisting calls to mandate masks on public transport and instead is rolling out a new app feature to warn commuters when their train is too crowded.

Transport Minister Andrew Constance announced the "world first" initiative on Tuesday, saying it would be crucial as people return to the office.

The Opal Travel app will use real-time data from registered Opal cards and loading patterns on Sydney trains to predict whether a commuter's usual train service is too crowded, and whether physical distancing will be breached.

A push notification will then be sent to the traveller's phone, on an opt-in basis.

While admitting the feature placed a lot of trust in users to make the safe choice, Mr Constance said he believed it would prompt them to reconsider their travel plans.

"I've got a lot of faith in the way the community have responded (to the pandemic) ... people have re-timed their days, people have stayed at home, people have opted (to use) active transport to get to work or drive a car," he said.

Employers would be understanding too, he said.

"It's going to be pretty hard for an employee to be angry at an employee for being 10 minutes late because they were avoiding a crowd on a public transport service."

The government has again stopped short of mandating mask use on public transport, despite calls from transport unions and epidemiologists, with Mr Constance pleading with commuters to wear masks voluntarily as patronage increases.

"Not enough people are wearing masks ... I just caught a train on the Eastern Suburbs line and nobody had a mask on," he said.

"We don't want to have to mandate it. We don't want to put in place measures, be it fines or denying people service."