A father told a Brisbane court the stress of quitting smoking was the reason he raped his 12-year-old daughter.

The "loving, caring and generous" man admitted he "crossed the line" when he sexually assaulted his eldest child after she asked for a quick cuddle before going to bed.

News of the man's attack was only revealed when the girl told her mother during a birthday celebration, the court was told.

When questioned by the mother, the man, who cannot be named to protect the daughter's identity, claimed giving up smoking had left him feeling "woolly" in the head.

He went on to admit raping their daughter had overstepped the mark.

Tim Meehan, solicitor for the defendant, said his client's "30 seconds of madness" was a "one-off, one-instance offence" that would not be repeated.

"(My client) is a loving, caring and generous father," he said.

The man, from Brisbane's northside, was jailed for two and a half years, suspended after serving 10 months, after pleading guilty to rape.