PASSENGERS should be able to pay bus drivers a ''premium'' fare to prevent increasing numbers of travellers from being turned away from Sydney's PrePay services, a transport lobby group says.

The call to allow casual bus users to pay for tickets bought from drivers on PrePay routes comes as State Transit expands the service into the inner west tomorrow.

Bus stops will go cashless - passengers have to buy tickets from participating retailers before they board - on weekdays between 7am and 7pm along Broadway and City Road as well as sections of King Street, Enmore Road, Parramatta Road, Norton Street and Glebe Point Road.

But Action for Public Transport secretary Allan Miles said catching a bus in Sydney was becoming too complex. One-off bus travellers were confused about what services were prepaid and were being turned away by drivers who would not accept their money.

And many convenience stores and newsagents complain they quickly run out of PrePay tickets, adding to traveller frustration.