For the 19th consecutive month, more buses ran late last month compared with the previous year.
So far in 2012-13, an average one in four buses has been late.
This compares with one in five in 2011-12 and one in six in 2010-11.
January is usually the best month for buses being on time, mainly because there are fewer cars on the road and schools and universities are closed. But the punctuality rate has fallen from 91 per cent in 2010-11 to 88 per cent last year and 80 per cent this year.
A bus is considered late if it does not arrive within four minutes of a scheduled time.
Transport Minister Troy Buswell said there had been a dramatic increase in the Transperth bus network and the frequency of services in the past four years.
"More people throughout the metropolitan area have bus services running closer to their homes than ever before," he said.
"But we recognise congestion on our roads is an issue for the city and have a number of significant roadworks projects under way that will improve travel times for motorists.
"January was a particularly busy month for roadworks, with major projects getting under way while others continued.
"These projects cause some disruption to traffic in the short term - including to buses - but we believe the people of Perth realise that some short-term pain will result in long-term gain.
"Bus travel times are also impacted by the growing number of vehicles on the road as Perth's population increases."
But shadow transport minister Ken Travers said the latest figures were another example of the State Government's failure to plan and build a public transport system.
"It is not just cars but also buses that are suffering from our congestion crisis," he said. "The Liberal Party's piecemeal response to congestion is to use road-based solutions that will only add to it.
"Labor's co-ordinated Metronet plan will see buses making short trips to train lines that will then move people quickly across the suburbs to their destination."