Recommendations please.

i am planning a trip to Australia and New Zealand in December 2017/January 2018.

The outline plan is to spend the first week in New Zealand. I have been told that we should head to Queenstown/Milford Sound.

We will then fly to Australia. I need a recommendation as to where to spend the second week. Originally I was planning to go to the Whitsundays/GBR, probably staying at Hagman Island. But I have since been told that it is a bad time of year to go. Too hot and jellyfishy, apparently.

We will spend the final week in Sydney.

So my questions are as follows:-

Where (locations and hotels) would you recommend we stay in NZ?

Ditto re week 2 in Australia.

Which hotels would you recommend in Sydney?

We are 2 adults and 2 boys, who will be 9 and nearly 11.

Many thanks for any help you can give.