Nearly a third of Sydney's bus fleet has driven the same distance as the trip to the moon and back, says the NSW opposition as it slams the Baird government for only adding 13 new buses to the ageing fleet.

Of Sydney's commuter buses, 636 have clocked up more than 756,000km, the same amount of kilometres as a return trip to the moon, opposition transport spokeswoman Jodi McKay said in a statement.

Labor claim the bus with the most kilometres has clocked up 1,571,195km in its 23 years on the road and is still servicing Sydney's Kingswood area.

"Most commuters would be horrified to know that the bus that's taking them to work each day has been around for longer than we've had access to the internet," Ms McKay said on Monday.

But Transport Minister Andrew Constance defended his government, saying state transit has rolled out 60 buses this year and the fleet was improving.

"When Labor was last in office, there were more than 300 buses in the fleet that had travelled over a million kilometres. We've more than halved that by investing millions of dollars in new vehicles," he said in a statement.

"In this year's Budget we announced a $108 million investment in new buses which will serve some of Sydney's busies routes and add thousands of extra weekly services."