The ERA agreed that Mrs Grey had applied for the leave too far in advance of the birth of the baby - had she applied at a later date, she would have been granted the payments.
"Ms Grey's timesheet history establishes that she has satisfied this test," authority member Michele Ryan said.
"I consider her early application can be characterised as a procedural irregularity from which relief may be available. Ms Grey has acted in good faith and this is a case in which it is reasonable for the authority to grant relief."
Yesterday a spokeswoman for MBIE said it was still considering the determination, and therefore it would not comment on the matter.
She drew attention to the changes to paid parental leave that would roll out in April next year.
She said the changes would address the problems with adoptive parents accessing paid parental leave and current restrictions with the hours test for eligibility.
Earlier this year Jayne Susan Montford was declined paid parental leave when she was approved to be the "home for life" parent of a child in Child Youth and Family custody.
MBIE said she did not have a "view to adopt" and had not taken steps to legally adopt the child when she assumed care.
The authority said it was apparent that Ms Montford assumed care of the child with a view to adopt if and when his biological parents gave consent.
Johanna Peck was declined paid parental leave by MBIE when she filed incorrect documentation when she was adopting the baby her niece was carrying.
In July 2013 she told the ERA she did not file the correct adoption papers because she was told the baby was regarded as a relative, so documentation was unnecessary.
However, a grand-niece does not constitute a "relative" as defined by the Adoption Act, and MBIE declined the payments.
The ERA ruled Ms Peck assumed care of the child based on an honestly held, but mistaken, belief that he was a family member and no other steps were required to take the child home.
Ms Peck was eligible for payments, the authority said.