A 26-year-old man from Ngunnawal will be summonsed to appear in the ACT Magistrates Court over the graffiti attack, which took place in Mitchell, according to police.

The man, who has not been publicly named, was allegedly seen using a spray can to daub streaks of red paint and green letters - perhaps a sprayer's "tag" - on the side of a tram in the Sandford Street depot on Wednesday.

Police say he was spotted by security guards and challenged at about 3.50am. He then left the protected site, only to be spotted later in the area and arrested. How he got into the depot despite its fencing has not been disclosed.

"A short time [after the incident] police conducted a patrol in the Mitchell area and observed a man fitting the description supplied by the security guard," police said in a statement.

"The man was carrying a knife and a backpack containing cans of spray paint which police believe matched that used on the light rail vehicle."

The man has been ordered to face the ACT Magistrates Court on charges including criminal damage and possession of a knife in a public place.

The graffiti attack is the second incident of vandalism against the light-rail system before it's even started running with paying passengers.

In December, a tram was targeted only a few hours after it had been delivered to the network. In the wake of that incident, security was stepped up from the original two security guards and sensor lights.

The network is due to start taking paying passengers towards the end of April. Trams will then be in the depot overnight but also vulnerable to vandals during their day to day service.

Police and light-rail managers may worry if trams become a "fashionable" target of vandals who take delight in penetrating heavily fenced depots to "tag" vehicles in a competitive way.