The cost of providing buses to replace Sydney Trains services has ballooned to almost $40 million, amid claims the jump has been caused by an increasingly unreliable rail network.

The amount of money spent on private buses to replace trains increased almost 40 per cent last year to its largest amount in five years.

The state government attributes the increase to a combination of upgrades and maintenance, weather disruptions, as well as the threat of industrial action in January.

Sydney Trains' spending on replacement buses increased to almost $40m last financial year. Credit:Dean Sewell

The opposition, however, explained some of these factors to a failure to provide enough resources to run a new rail timetable implemented late last year.