Overcrowding on Melbourne's trains has worsened over the past year, with half of all services on the Craigieburn line crammed with more people than they should be during the morning peak.

Public Transport Victoria (PTV) figures show 20 per cent of all morning peak-hour Metro trains were overcrowded during its May survey, with the Craigieburn, Sunbury, Cranbourne and Pakenham lines bearing the brunt of the load.

The percentage of Craigieburn services breaching the load standard jumped from 40 to 50 per cent in the past year.

On the Sunbury line, 40 per cent of morning peak-hour trains were overcrowded, along with 37.5 per cent of Cranbourne and Pakenham line trains running along the busy Dandenong corridor.

Despite the peak-hour crush, the PTV report said no net additional services were added to the network during the morning peak in the year to May.

Public Transport Minister Jacinta Allan said the figures demonstrated the importance of the $11 billion Melbourne Metro Rail tunnel.

"It's further evidence as to why we just don't have a moment to lose in getting rid of those dangerous, congested level crossings on the Dandenong line, having the high-capacity trains run along the Dandenong line," she said.

"These are big projects that are going to provide a significant increase in the number of trains we can run for a growing city."

During the afternoon peak, the overall percentage of overcrowded trains dropped to 7 per cent.

Opposition warns overcrowding will get worse

But Opposition spokesman Matthew Guy warned the overcrowding was only going to get worse.

"We can build Melbourne Metro, we can build Metro one, two or three — it won't solve the problem if there are eight million people in Melbourne and we don't have a plan to decentralise our population," he said.

Public Transport Users Association spokesman Daniel Bowen said the Government would have to run more trains more often, well before the scheduled opening of the Melbourne Metro in 2026.

"The Metro tunnel will help a lot, but that's another 10 years away and I don't think Melbourne can wait that long to see the overcrowding on the trains relieved," he said.

Mr Bowen said it was disappointing the last major timetable change was in 2014.

The Melbourne Metro will connect the Sunbury and Cranbourne-Pakenham lines via a new tunnel through the city.

Early work to move utilities to make way for the tunnel has begun, but the Melbourne Metro Rail Authority insisted disruption to power and gas to homes and businesses would be minimal.