Auckland: 1 day (2 at the very most). The rest of NZ: 20 days. I know that leaves 0 for Australia, but you asked for my opinion. Actually it depends what you want to see; NZ is beautiful country but lacklustre city (however Auckland is still a nice city by world standards, just not really worth a special trip). Many other places in NZ are worth a special trip, but it is a country of great diversity and you couldn't see it all in 20 days. Ditto for Australia.
A serious recommendation: 6 days in NZ, 14 days in Australia. For the NZ portion stick to the North Island, there isn't time for the South Island. Spend the days as follows: Auckland, Thames, Rotorua, Napier, Turangi (Lake Taupo), Auckland (best done renting a car but can be done by train/bus). A bit hectic (drop Napier if it is too much, that is the least interesting part of the circuit).
While in Auckland be sure to visit the Museum in the Domain (contains the world's largest collection of Polynesian artifacts).