The woman, whose name is suppressed, was found guilty in a reserved decision delivered by Judge Lawrence Hinton this week, Stuff reported.
The baby, born in 2016, had twice tested positive for methamphetamine when it was 3 months old.
The first time was after it became unsettled and was admitted to Hawke's Bay Hospital.
The second time was after the baby was readmitted nine days later, as it was stiff and unresponsive, Stuff reported.
Police charged the mother with a representative charge of "intentionally engaging in conduct likely to cause suffering or injury, or adverse side effects to health of the child, such conduct being a major departure from the standard of care to be expected from a reasonable person".
She will be sentenced next month.