An advocate for free public transport in Auckland believes the benefits would far outweigh any cost of setting it up.

Hamilton's mayor wants all buses to be free, all the time, following a free bus trial for under-18s over the weekend.

Auckland Council's Roger Fowler told Tim Dower it would cost a lot to set up free public transport, as services would need to be improved.

But he says no one seems to bat an eyelid over the cost of new motorways.

"You raise the question of making public transport a decent public service and attracting people onto it, some people throw their hands up in horror."

In Auckland, public transport will be free for five to 15-year-olds at weekends from September.