A photo of Mr Gallagher was released to the public on Thursday before he was arrested today at 7.45am.
Gallagher appeared before Central Local Court on Monday charged with stalking and intimidation with intent to cause fear as well as trespassing and common assault.
An email circulated to ABC employees, seen by AAP, says Gallagher is believed to have tailgated staff walking through the building's high-security doors. There is now an increased security presence at the Ultimo office, the email states.
The 31-year-old was detained while his mental health is assessed.
He "appears to have significant mental health issues" and "became abusive" while appearing via video link, court documents reveal.