At issue are National Party membership targets set by the party's board for each electorate.
Those electorates are challenged to hit a membership target before they can select their candidate for the next election.
The targets are different for each electorate, reflecting the different dynamics of each seat.
Grigg's Selwyn, a very safe National seat, would have a different target to neighbouring Wigram, a safe Labour seat held by Labour's Megan Woods.
If Grigg fails to hit the target, a selection will still happen and she would likely be reselected anyway, unless a popular opponent mounted a challenge.
Grigg did not comment for this story, but a National Party spokesperson said that the party Board "sets a stretch membership target for each electorate to meet before going to selection".
They said that in Selwyn, this target "will probably be met in the coming weeks".
National does have a rule that in electorates with fewer than 200 members, the board runs a selection contest instead of local members, however a National spokesman confirmed that "Selwyn has well in excess of 200 members and will be a local selection".