It is not just about airbags being contained in the seat itself, it is also about the one that is in front of you (being in the steering wheel or dashboard on passengers side. One of the criteria LVVTA states for seats is that any vehicle fitted with driver or passenger airbags must maintain the same relative position between the top of the seat cushion of the original seat, and the top of the seat cushion of the retro-fitted seat. This allows the factory fitted airbags to still be effective at protecting you in an accident. rather than snapping your neck if you are sitting too low.
so technically you will need a cert.
check out the LVVTA standards below if you are unsure: http://lvvta.org.nz/documents/standards/LV..._Anchorages.pdf
If after reading that you are still not sure, consult your local LVVTA certifier, not an internet board. LVVTA certifier is the one making the decisions, not us ;)