Hi Mike
My husband and I have 7 kids ranging in age from 17 down to 3 years old. We've been on some great family holdiays over the years but as the family grew the choices became limited because not many places can cater for such a large family or the activities may suit a certain age group but not another. Even though there are not many families as large as ours there are a lot of blended families that holiday together and as such would face similar problems as we do.
My suggestions from experience are:
1. Broome, WA - great beaches, great outback places to see, good food and people but may be a bit restricted in activities for teenagers.
2. Tasmania - Hire a troop carrier and visit the many fabulous locations, Cradle Mountain, Freycinet, Hobart, Launceston, etc. The kids loved the trip across on the boat - recommend bungey straps for little ones!! A bit nerve wrecking for Mum !
3. Kingfisher Resort on Fraser Island or one of the other resorts, great 4WD, pools, nice facilities and lots to do for all ages.
4. Lindemand Island - Fantastic Circus school suits all ages, packages include all you can eat food, beer, wine and soft drink. But a bit pricey when your family gets to our size, we got a quote recently to go back there - $20,000. We'll give that one a miss.
5. Couran Cove - all round good spot.
6. Gold Coast - but I think it's been done to death and wouldn't go back there except maybe to Dream World or Wet 'n' Wild.
7. Sunshine Coast, QLD - we enjoyed this spot so much we now live here. The Big4 caravan parks have good self-contained lodges, we got two side-by-side which the older kids liked having a place of their own away from the little ones.
8. But my absolute all time must go back to loaction is for Mum and Dad only - Orpheus Island - devine, my husband and I hope one day to be able to go back there, fantastic, luxurious and pampering. Makes having kids all worthwhile if you can get a break at a place like this.
Overall, simpler is better, don't sweat the little stuff, the kids have fun just going somewhere, having nice food and a few extra treats can be novelty enough for most. Experience what your own local areas have first, check out the national parks and other interesting locations. you might be surprised how much is on offer close to home.
For the teenagers ensure you've got internet access and mobile phone coverage or they get sever withdrawals !!! We always bring a laptop and mobile phones with internet access, also useful for checking out what's on and places to go.