A cow that fell off the back of a truck in the Domain Tunnel is causing a major peak-hour headache for Melbourne commuters this afternoon.

A VicRoads spokeswoman said three of the tunnel's four city-bound lanes were blocked and there were traffic delays as far back as High Street, Glen Iris.

Motorists going home on the Monash Freeway also face delays, because of a collision on the eastern side of the road near Belgrave-Hallam Road, Hallam.

The accident happened about 4.20pm.

VicRoads said traffic was already heavy from the approach to Stud Road, Dandenong.