Wow that is shit. Poor you. If it makes you feel any better, and I doubt it does, you're not the only one, I was left without a pot to piss by my lovely x! but ... things are getting better.
I think you could represent yourself. You need to be aware of all the worst case scenarios though. If he didn't return the children, how supportive would the Australian police and courts be?
I think your starting point is a really strong case though.
Why is he going to Australia? If he is not actually Australian himself, then it will be viewed even less sympathetically. IF he is Australian and born and brought up there then the courts might have some sympathy with him. Estranged father with no family support... but if he's British and he is choosing to remove himself voluntarily from the children's habitual domicile then it won't be viewed all that sympathetically I'm guessing.
When you say his parents could accompany the children to Australia, I'm guessing his parents at least are based here, so...
Does he have a new girlfriend? Is she Australian? If so, he's prioritising a relatively new romance over the quality of his relationship with his children.
Doing a bit of a Brian McFadden!! by the sounds of it.