As far as the Sec. 18a goes, the 3 months starts from the granting of the PR permit. You would initially be granted a residency visa so your permit will be granted once you enter NZ and the clock starts running.
Dratted pdf file won't allow me to paste it here. You'll find it
here about half way down the document under Section 18A Requirements. It says more or less the same as the below which I already had on my file.
Where a residence permit(s) is granted under the Skilled Migrant Category on the basis that the principal applicant* qualifies for points for an offer of skilled employment in New Zealand or current skilled employment in New Zealand for less than three months (see SM7), the holder(s) of the permit is subject to the following requirements:
In the case of the principal applicant* who:
has an offer of skilled employment, they take up that offer of skilled employment within three months of the grant of the first residence permit, and they remain in that employment (or another position of employment that meets the requirements for offers of skilled employment including requirements for bonus points if the offer of employment qualified for bonus points under SM8 or SM9), for a period of at least three months;
or
has current skilled employment in New Zealand for less than three months, they remain in that employment (or another position of employment that meets the requirements for current skilled employment including requirements for bonus points if the employment qualified for bonus points under SM8 or SM9), for a period of at least three months; and
they inform the nearest branch of the NZIS of their residential address and any changes of residential address while they are subject to those requirements; and
they submit evidence to a visa or immigration officer that, within seven months of the grant of the first residence permit, the requirements set out above have been met.