The cost of Labor's 12-month trial of all-night public transport on weekends has blown out more than $30 million because the government failed to factor in the cost of extra security.

The Andrews government says the trial will cost $83.6 million, not $50 million as was promised, with the need for extra protective services officers and police adding $34.1 million to the cost of the "Homesafe" promise.

The extra protective services officers will be stationed at premium stations only, not at all stations.

Melbourne's trams will run all night every 30 minutes on six "key" routes with trains to run every hour across the network as part of a trial of 24-hour weekend public transport.