I think in the month of August anywhere north of Delhi would be good. Ideally agra , Rajasthan may seem great but will be very very HOT AND HUMID and one does not enjoy the place if the weather is not right.I would recommend coming upnorth to HImalayas......probabaly Spiti ( check out ecospherespiti.org ) or try Manali / Dharamsala. You may also visist Ladakh but i presume Spiti is your best bet.
Useful replies already. Take care of weather [it must be cold there in NZ?] and visit only places around Delhi - mostly cultural and historical sites. However, if u want to have a taste of Indian hill stations, go to Nainital on a short visit. It is more beautiful than some advised before.
See that you've got a lot of nice replies.
One additional advise - time. remember, everything takes time in India. Plan with a leeway of 30% extra time for everythings.
Have a safe journey.