About one in five Transdev buses ran more than five minutes late last year. The bus operator is responsible for a third of Melbourne's bus network.

Transdev has never met its contractual monthly punctuality target and has overseen an annual decline in ridership of tens of thousands of passengers on key routes.

Its overall on-time performance of 80.7 per cent is lower than Metro Trains, Yarra Trams or V/Line.

Transdev's results have not been released publicly. The Age had to use freedom-of-information laws to obtain them.

The data showed that last year, just 14 of Transdev's 46 routes hit the 85 per cent punctuality target signed up to by the company.

Routes that missed the mark included the three outer-suburban orbital smart buses - which were used by a combined 12.7 million passengers in 2016 - and most of the Doncaster area buses that serve Manningham in Melbourne's north-east, which has no trains or trams.

The worst performer of all was bus route 220, a busy cross-city route between Sunshine and Gardenvale, which ran just 67 per cent of buses on-time.

Transdev is at the halfway mark of a seven-year contract that Public Transport Victoria said would usher in a "generational transformation" of Melbourne's under-utilised buses, with better and more reliable services and increased passenger satisfaction.

Instead, it has mostly failed to meet the performance benchmarks in its contract, while its key proposal for improving performance - a new bus network with redrawn routes and timetables - has been blocked by the Andrews government.

Transdev was supposed to increase passenger numbers 5 per cent a year, but they have fallen significantly on many of its routes. For example, Melbourne's most used bus route, the 903 between Altona and Mordialloc, carried 5.18 million people in 2016, down from 6.03 million in 2014-15.

Melbourne's buses are the only public transport mode that has experienced a recent fall in passenger numbers.

One Melburnian who catches Transdev buses to and from work is architect Elise Houghton, who commutes each day via route 905 between the city and Templestowe.

Ms Houghton became so frustrated by late or cancelled buses that she complained to the Public Transport Ombudsman in November.

"I'd just returned to work as a new mum in my dream job and I was pretty keen to impress my employers," she said. "Working part-time hours, there was only so many times I could use the excuse that my bus didn't turn up, and it got to the point where it was embarrassing for me."

The government and Transdev negotiated a series of minor timetable adjustments in June, which triggered the 85 per cent performance target.

In 2013, the company won a competitive tender to operate many Melbourne routes in 2013, and signed a contract that included fines and bonuses based on punctuality, reliability and patronage targets, a unique arrangement among Melbourne bus companies.

Transdev admitted through the Ombudsman that it had struggled to run a reliable service late last year because it had a shortage of buses and bus drivers.

Ms Houghton said reliability had improved slightly since then, though she was still heavily delayed about once a week on average.

Public Transport Users Association president Tony Morton said Melbourne's buses struggled to run on time because they were not given priority on the roads, and struggled to attract passengers because they were infrequent and indirect.

"If we designed bus routes to grow patronage ... to make it work for everyone getting from A to B, just as trams work in the inner-city, more people would use them," he said.

Transdev said traffic congestion was the main cause of its buses running late.

"We continue to look at ways to improve our operations that will lead to more punctual and reliable services, and we were able to make some small timetable changes last year that took current traffic conditions into account," spokeswoman Kate Rasmussen said.