Hello
You will not be able to get one unless you meet one of the three criteria found in WD3.1.1 f i ii or iii
Applicants may be granted a work visa if:
f) they have not previously been granted a Post-Study work visa unless:
i)they have undertaken and completed a second higher qualification that is either a New Zealand Bachelor’s Degree or postgraduate qualification, and have studied that qualification full-time in New Zealand for at least 30 weeks; or
ii)they are working towards occupational registration and meet the requirements at WD3.1.15; or
iii) transitional provisions apply (see WD3.5).
So as it appear you wouldn't be transitional (Those that were already on SV's when the rules changed).
You would either need to do a teaching qualification to meet ii) or a Bachelor's degree or higher (Post Graduate, Masters, Doctorate).
A Level 7 Diploma would not be high enough to get another Post Study Visa.
The table in WD3.1.5 shows that INZ makes a difference between a Bachelor's degree (L7) and a Diploma (L7)