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A bus driver who refused to let a child on board because he had $20 to pay his $1.80 fare should never have left that passenger behind, unions representing drivers say.

Kay Pridmore was issued the warning last October by her Tauranga-based employer, Go Bus Transport after leaving the child behind and then later telling her supervisor if she wanted to give change she would work in a "f***ing bank".

NZ Tramways Union spokesman Gary Froggatt said drivers were never supposed to leave children behind.

"That's a real no-no because there's a risk of something happening to them."

The passenger would be told to take a seat and the driver should contact the control operator for advice.

"Usually the advice would be to take the passenger's name and address and the company would recover the fare at some stage."

But he said some passengers had the drivers on on a daily basis, turning up with $50 or $100 notes.

"That makes it impossible for the drivers to change because the drivers are not carrying a lot of cash now because of the electronic tickets."

First Union Transport/Logistics spokesman Karl Andersen said one of the companies it represents, NZ Bus, had the same policy of not leaving people behind.

But problems arose if a bus inspector found a passenger without a ticket, he said.

"The inference taken is that the driver has taken money off them and not issued a ticket and put the money in their pocket."

Supplying drivers with a larger kitty presented safety issues, he said.

"But they are told that they should not refuse someone entry to the bus because they've got a large note that can't be cashed."

Nobody was immediately available for comment at Go Bus.

Ms Pridmore took Go Bus to the Employment Relations Authority (ERA) because she said the warning she received about her behaviour unjustifiably disadvantaged her employment.

She said after telling the boy he needed smaller currency to pay for his fare, he walked away and had not returned by the time the bus was due to leave.

Her supervisor, Paul Devoy, was made aware of a complaint about her actions and informed Ms Pridmore it was her responsibility to take the boy.

Ms Pridmore said she was furious for being blamed for the boy not being given passage to the city.

In a phone call later that day with Mr Devoy, Ms Pridmore immediately became abusive, saying to him: "I am not a f***ing banker, if I wanted to give out change, I would work in a f***ing bank".

Go Bus investigated the incident and issued her with a written warning.

In a decision released today, ERA member Eleanor Robinson said Ms Pridmore should have had plenty of money to give change to the boy, but even if she did not, company policy was always to give children passage if the driver did not have the appropriate fare.

"In these circumstances I determine that the decision to issue Ms Pridmore with a written warning in respect of the allegations had not constituted an unjustifiable action."

However she recommended the company issue guidelines on how drivers were to deal with $20 notes.

Costs were reserved.

- APNZ