3 weeks in Australia with 4 year old

[JG24]

Can anyone help me plan our holiday please?
In 2 years we (me, partner, 4 year old dd) are going to go to Australia to visit my sister.
The plan is to fly to Sydney with my 75 year old dad and spend a week with my dad and sister in and close to Sydney (where she lives).
Then ideally we'd leave my dad and sister together for 2 weeks whilst we (me partner and DD) spend 2 weeks together. Then we'll come back to Sydney to get my dad and fly home together
Any ideas what we could do in those 2 weeks that's suitable with a 4 year old?
We were looking at maybe taking a campervan down to Melbourne for 5 days?
I've never been to Australia
My partner has but years ago
Partner is fairly adventurous (likes surfing, kayaking etc) but obviously we're a bit restricted having our daughter with us
Any tips appeciated
Thanks all

So this trip isn't for 2 yrs/
What time of year would you be going? What things do you like to do?

There's lots of things to see and do in all of the major cities and in between that a 4 year old would enjoy.
Melbourne is great for kids (we recently spent a long weekend with our 3 and 8 year olds and they loved it).
I think I'd try and work out what you and your partner really want to see/do and then factor in things for the 4 your old along the way e.g. there are some great playgrounds, museums, gardens, beaches, bush walks.

Personally the thought of doing any kid of campervan road trip with my boys is the stuff of nightmares (can't go 5 minutes without them winding each other up) so I'd want to plan in lots of stops - there are so many lovely places to explore all the way down the coast.

I'd be tempted to do something like this, a trip taking in lots of places and includes an overnight train ride.

www.tripsbeyond.co.uk/holiday/australia-by-rail/

We're currently thinking may.
Before our daughter we would usually spend 3 weeks in a country going to 3 or 4 different places - cities/beaches/villages.
We're probably less of city people, we like playing on the beach, seeing animals, site seeing, food

I just spent 2 weeks in VIC with my 4 year old and she had a ball. Helped that she was meeting a large, welcoming family for the first time.
We did a Sydney overnight and she wasn't thrilled because there were no parks at the Harbour 🤣 she enjoyed the Manly ferry and beaches there though.

We didn't venture into Melbourne city. I lived there 10 or so years ago and couldn't think of anything that would entertain her.

If you're going to hire a camper I would recommend driving down to Geelong and playing along the waterfront then head down the Great Ocean Road as far as you can in the time. Drive back inland and visit Ballarat for the wildlife park and gold mining.
The Queenscliff to Sorrento ferry is also a nice day out. There's an aquatic education center in Queenscliff but you need to pre book.
DD was just really happy with beaches and parks

Just bear in mind if you do come in May, its late autumn in Victoria, temperatures during the day can be pleasant but its cold overnight.