What do you mean by "new administration"? Do you mean if Australia has a change of government? (The term "administration" is used in presidential systems like the US, not parliamentary systems like Australia.)

Both major parties in Australia are big fans of surveillance so I don't think a change of government would make any significant difference to Australia's surveillance laws. The police and intelligence agencies start up a chorus of "we need this to stop terrorists and pedophiles" and both sides of politics reply "of course! of course! whatever you need!"

I don't think the legal situation for Fastmail in Australia is hugely different from that in other countries – look at national security letters in the US, bulk collection warrants issued by the Foreign Intelligence Surveillance Court, etc. Yes, the Australian government can demand information but the same is true in many other countries. The most disturbingly unique thing about Australian laws is the government could – at least in theory – order a company to circumvent encryption or insert spyware into their product's code. Given Fastmail has all the data in cleartext, and it is server-based, I don't think Fastmail has to worry about such orders from the Australian government, they aren't relevant to Fastmail's product.

But if Australian law does become a big issue for their business, they could always relocate elsewhere.