Public transport users at about 100 train stations and ferry wharves will soon be able to top-up their Opal cards from machines.

The government plans eventually to introduce top-up machines to most - but not all - train stations across Sydney, and there should be at about 350 machines across the city by early next year.

Public transport users will soon be able to top-up their Opal cards from machines.

But by mid-August there should be machines allowing users to add money to their Opal cards using credit or debit cards at 96 train stations and wharves, as well as the Bondi Junction bus interchange.

Machines that will allow users to top up their cards using cash will be introduced later this year.