The shortfall was causing the most issues on weekends, with 11 per cent of weekend trips dropped since the new network launched.
The council was considering reducing weekend services and imposing financial penalties until the problem was fixed.
Tauranga-based First Union organiser Graham McKean, representing about 70 bus workers in Tauranga, said local working conditions made the job unattractive and had caused some new recruits to quit.
When the contract was announced, the council said drivers would be paid the living wage.
McKean said while that was more than the previous operator paid, NZ Bus had set the rate at the 2017 living wage of $20.22 for the duration of the contract, and was reluctant to have it evolve as the living wage did each year. The current rate is $20.55.
Living Wage Aotearoa convenor Annie Newman said it was "incorrect" for an employer to describe a wage as the "living wage" if it did not move with the annual recalculation.
CBD congestion, busy interchanges, long shifts and split shifts were other issues.
McKean said drivers were working the morning peak then had three or four hours of unpaid downtime before working the evening peak.
That meant those that lived too far from the depots in Greerton or Pāpāmoa to go home in the break effectively had to commit almost 12 hours a day to work.
Speaking on the condition of anonymity, one driver said returning home in the break was pointless as it would be an hour round trip, so the downtime was usually spent "twiddling my thumbs" at the depot.
Another said the lack of a base for drivers at the main CBD interchange in Willow St meant drivers had to use public toilets "fouled up by homeless people to the point where we have to walk in the stuff then traipse it back on to the bus".
That driver was looking for other work.
NZ Bus chief operating officer Claire Neville said the company generally offered the best terms and conditions in the areas it operated.
The driver shortage was a national problem, reflecting a difficult labour market and close to zero unemployment.
While "a small number" of new recruits had quit before completing training, she said the problem was not turnover but attracting enough candidates in the first place.
She said broken shifts were a "reality of the industry" and some people liked having time off in the middle of the day.
"Bus driving can be a great job especially for those who enjoy serving the community and interacting with people."
Regional council transport manager Garry Maloney said his team was working with Tauranga City Council around issues in Willow St.
The new network was being monitored and he expected tweaks would be made in late March.
