One card that Australia still has up it's sleeve is iron ore. China needs it, and Australia is the only country that can supply it in the quantities that China needs.
Now the question is, does Australia really have no cards to play? Iron ore being the sole trump card is a pretty weak hand...
Don't forget milk!
Seems the two cases are related - they wanted to question them about Cheng Lei. Australia has no accredited journalists in China at this point - extraordinary.
Quoting from the FT story referenced in the tweet.
The Australian Financial Review reported that both journalists were asked questions about Ms Cheng by state security officials.
On Tuesday, China’s foreign ministry confirmed Ms Cheng was “suspected of engaging in criminal activities endangering China's national security”, without elaborating. It added that the questioning of Mr Birtles and Mr Smith was conducted “according to law” in the context of a “normal law enforcement action”.
If you are small open economy selling commodity items, you sell the good on the open market for the next best price. It's not like they customized the good for China and China was the only market in the world for those goods.
You can always slap a 20% import duty on made-in-china Goods, or add some new random tariff import rule, so do more "random" spot checks. Or force all made-in-China goods to be loaded in some boon-doggle port, with no deep water access, no rail and a dirt road for freight trucks and only put one custom inspector that needs a seeing eye dog, works half days, etc.
Then your Australian retailers source their stuff from other-places than China, say Vietnam, Bangladesh, India, etc. Start yet another trade-war and ding your own export suppliers. Swings and round-abouts. China goes complains to the WTO, drags out for a few years... Australians don't generally play that game, but Trump and China has opened up a Pandora's box of retaliatory trade measures.
I don't think Hong Kong has approved the Australian consul-general for like 6+ months... No visa approval.
Australia meanwhile, like any democracy, is driven by loads of different interests. Mining probably isn't even the biggest one right now, education might be. Chinese tuition fees made Australian universities rich. At the same time, academia is the perfect place for China-bashing - anything western academia values is suppressed in China
TL,DR - it doesn't matter what endgame people have in mind. What is happening is largely out of their hands.