The 28-year-old Malaysian school teacher charged with criminal trespass in Singapore has a long history of mental illness, said a relative.

Quoting her aunt, Madam Hanifah Che Pi, Malaysian paper The Star reported on Friday that Nurul Ruhana Ishak had to be on medication constantly or she would suffer from depression.

Nurul is the eldest of five siblings from Pendang, in Kedah state, and suffered from depression in school and during her university years. Despite her illness, she graduated as a teacher from Universiti Sains Malaysia in 2010.

She was arrested on Monday in the compound of the Ministry of Foreign Affairs, and is being remanded at the Institute of Mental Health. Said Ms Hanifah, a housewife: "When she is ill, she becomes quiet and has to be spoonfed, but when she is feeling alright, she is a sweet and intelligent girl."

Her niece had allegedly slipped past immigration officers at the Woodlands Checkpoint last week by tailgating a car. An alarm was raised two minutes later and the arrival zone locked down, but her red Malaysian-registered Perodua hatchback had gone by then. Investigations into her border intrusion are ongoing.