You do have mealys...but that much eaten frond is more mollusc. Bait for slugs and snails,or go out at night when they are active.

Third- you are in Australia...so maybe one of the Marsupial species are nibbling?

Okay thanks guys! I'll throw some snail pellets down now and grab some spray for the mealy bugs when I get home tonight.

There's new grown opening up just now on the plants, so I've chopped off all the manky fronds.

We have actually had some possums fighting in the garden the last few nights, so that could also be it too.

Those are from butterfly larvae. There are a number of species of Hesperids (Skippers) that feed on certain palm species. The caterpillar curls up the leaflet around itself and attaches a silk thread (as shown in your photo) to keep it in place. It then eats the leaflet down to a point where it must abandon that shelter for a new one. The cycle keeps repeating itself until the caterpillar matures, or is taken out by a predator. Possums don't usually chew on palm fronds, nor do snails. Since your palms are still small the easiest way to get rid of the caterpillars is to just squash them in their 'tubular homes'. Cutting off the fronds will set the plant back. While it may look unsightly, any green leaflets remaining are producing food for the plant.

Cool, thanks tropicbreezent, that explains a lot. I had noticed a couple of weeks ago that a lot of the leaves had been curled up, even though the plants looked otherwise healthy.

Interesting that they only seem to like the bangalow palms, and haven't touched the kentia palms at all.

There's new growth about to open up any day now, so I ended up cutting off almost all the existing fronds.
I've got some stuff that says it gets rid of caterpillars, so I'll start spraying the plant with that to try keep it safe in the future.

Well,you might just cut off the offended fronds. I don't think you really want to poison the Skippers.

They are so much fun to watch in summer.