I'm just back from 4 weeks visiting my family with my six month old and had a total ball. I'm very jealous!
In terms of sunscreen, it's not quite true that you can't buy high-protection sunscreen there. Australians take sun protection incredibly seriously, but by law sunscreens can't be labelled as higher than SPF 30+. Anything you see that's labelled 30+ will probably be at least the equivalent of the SPF 50 you get in the shops here.
I'd also look into a full sunshade for your pushchair - it made a lot of difference to us, particularly in terms of naps in bright sunshine.
Be prepared for the weather to be quite changeable in October. In some places it'll feel like a hot British summer but in others (particularly Tasmania) you may encounter some quite cool weather. It's a funny time of year but I always think April and October are the nicest times of year to visit, weather-wise.
Our (very tall) nearly 7-month-old only just fit in the bassinet on the plane on the way home but it's worth requesting the seats with a bassinet anyway. DS loved sitting up and playing in the bassinet which was a godsend during meals. They allocate the bassinets based on the ages of the babies so you may not be lucky if there are lots of smaller babies.
We invested in a couple of packs of novelty-shaped post-it notes for the flights (we had a couple of internal flights as well) and they were brilliant stuck all over the seat in front, the tray table, Daddy, etc. I'm not sure what age you can get headphones for babies/toddlers but DS was entranced by the TV screens. The downside was that they were touchscreens and he kept changing the programme!
When driving around, we did a lot of our longer-distance driving after bedtime which helped.
To be honest the flights weren't as bad as I'd expected but I went in with the mentality that it would probably be awful and we would just have to survive it! The baby jetlag afterwards was pretty awful (we are still dealing with it four days after getting home) but interestingly DH and I dealt with it much better than usual, I suspect we're better at dealing with disrupted sleep these days!