Hi Hiromi,
Will you have a car? Ít's lovely to drive through the wine country and just have lunch outside at one of the vineyards such as Matakana's Ascension or Kumeu's Soljan's. Are you planning to go to Waiheke - my favourite restaurant in NZ is this one:
www.tewhau.co.nz
Also good is
http://www.hammerheads.co.nz/ a 5-10 minute drive from the city centre - great views and great seafood. Their chowder is to die for.
Sorry, don't have any tips for Rotorua/Tauranga.