I need an additional 100 status credits to retain Qantas Platinum

I need an additional 100 status credits to retain qantas platinum - how can I achieve this using the least amount of points/dollars?

Look at Qantas Any Seat flights, which can use a mixture of points and dollars or just straight points.  This will earn you FF and SC.

This article may help with the points+pay Any Seat Award scenario, Sandy: 

http://www.ausbt.com.au/expert-tip-save-frequent-flyer-points-on-qantas-award-seats

Many thanks for this link it was extremely helpful!

Alternatively, you could fly to AKL for the weekend using cash and it won't set you back by too much. QF is having Business class at 500+ and 200+ for Economy. One way on buisness will give you 80 SC and you can return on Economy which gives you 20 SC. It will cost you about 750 all in for 100 SC.

Or the same with SYD-MEL business, along with Economy back.... You should have received the Double Status credit offer from QF being a platinum, so that should take you over 100SC.

Does Qantas do family pooling like Virgin? My friend was about 100 SC away from achieving Plat so he booked a family weekend to the Sunshine Coast, he got Plat membership and had a great weekend away with the family.

QANTAS does not do family pooling of SC.  You can do a once per year transfer of points only to immediate family of up to 100000 FFP.

Where are you based?  Have you rec'd a Double SC offer from QFF?