Mr. Wemmick does not actually say that his employer is "deeper" than Australia. The exact words are:
"Deep," said Wemmick, "as Australia." Pointing with his pen at the office floor, to express that Australia was understood, for the purposes of the figure, to be symmetrically on the opposite spot of the globe. "If there was anything deeper," added Wemmick, bringing his pen to paper, "he'd be it."
The suggestion is that if it were possible to dig a deep hole through the earth from the vicinity of Jaggers' office, one would come out in Australia on the opposite side of the globe. Australia was the British colony to which many convicts were being "transported," and it was generally considered a terrible form of punishment to be sent there, only slightly better than being hanged. It was so far away and took so long to get there by sailing ship that it would be comparable to transporting convicts to Mars at the present time--and there must be science-fiction stories in which there are futuristic prisons on distant planets.
Wemmick is Jaggers' confidential secretary. He knows that Abel Magwitch, who is one of Jaggers' clients, was transported there and that he has been sending money to transform Pip into an English gentleman. He may be enjoying referring to Australia without revealing anything about Magwitch to Pip. There is just the slightest bit of foreshadowing in Wemmick's reference to Australia, and Pip may remember it later on, after he has met his benefactor and realizes that all the money he has been spending was coming from that distant penal colony.
Wemmick admires Jaggers and is devoted to him. He lacks Jaggers' intelligence and "depth," but he seems to be endeavoring to emulate his employer to the best of his limited ability. He enjoys the privilege of being in possession of secret knowledge. He enjoys mystifying people--as he does in Chapter 55 when he pretends that his meeting with Miss Skiffins is accidental and that their wedding was not all prearranged. Wemmick is not exactly an imitator of his employer but more like a reflection of a man who is so dynamic that his personality rubs off on anyone who associates with him for any length of time. When Wemmick says that Jaggers is as deep as Australia, Wemmick is trying to be deep himself.
There are many things that Jaggers does not want to know. When Pip first visits his office he hears Jaggers tell "two secret men":
"I want to know no more than I know."
Jaggers is technically an officer of the court and has to reveal anything he knows about a case--but he can't reveal anything he doesn't know. It is probably a great convenience for him that in Wemmick he can have a clerk who can know things Jaggers doesn't know himself--and doesn't want to know. But compartmentalization of knowledge is common enough today in politics and big business and certainly in the legal profession.