He has already asked Mr Key for a public apology, which the National Party leader has rejected.
If the court rules that the conversation was not private, meaning the recording was not illegal, it could strengthen a possible defamation case.
The teapot tape dominated coverage of the election and increased scrutiny on the future of Act last week. In the chat, Mr Banks is alleged to have agreed with Mr Key that Catherine Isaac, number two on the Act Party list, should be party leader.
Act leader Don Brash yesterday did little to quell speculation that he might not last a full three-year term if elected.
Asked if he might step aside for Ms Isaac if only two Act MPs were elected, he told Radio NZ: "I'm not going to make a decision at this point.
"Catherine Isaac is a first-class person, in my judgment. The [party] board put her at number two on our list, indicating clearly that she is a potential leader of the party, and I agree with that. I'm simply not going to speculate what's going to happen after the election."
Dr Brash has told the Herald he expects to remain leader for a full term, but added that the incentive to stay would diminish if Act had little leverage in the next Parliament.
He has not ruled out taking up an overseas diplomatic post.
Such a post was raised with him after National took office in 2008, but he declined.
Mr Key last week said he had not had any recent discussions about Dr Brash taking up such a role "to the best of my knowledge".
