Some of the flowers withered. Is this normal after blooming season?
Yes. Flowers will die once they have completely their life cycle.
Here is an article that will help you with the care of the plant:
https://www.gardeningknowhow.com/ornamental/flowers/wishbone/growing-wishbone-flowers.htm
Check with your local nursery to see if they carry it or can get it for you. I checked a couple websites of NZ nurseries, and no, they didn't carry it. You can order seeds online, they should be fairly easy to grow.
https://www.gardeningknowhow.com/ornamental/flowers/wishbone/growing-wishbone-flowers.htm
Fertilizing and deadheading will be key to keeping the plant blooming, successively:
https://www.gardeningknowhow.com/ornamental/flowers/wishbone/growing-wishbone-flowers.htm
My wishbone plant dropped seeds that are now growing...only an inch or so high and already have some flowers. Is this unusual for the plants to flower already? Please explain the process!
Many plants will reseed easily, especially if they need light to germinate, such as with wishbone flower. Sometimes reseeding annuals won't return till the next spring, so torenia is extra prolific. It is an annual that, starting from seed, grows quickly, blooms prolifically, then goes to seed.
https://www.gardeningknowhow.com/ornamental/flowers/wishbone/growing-wishbone-flowers.htm