A not guilty plea is not part of the case of a defendant or a defense attorney. A "case" refers to evidence and argument made at trial (or conceivably in a pre-trial hearing). The rule in question specifically contemplates a defense attorney entering a not guilty plea for a client who has confessed to the lawyer that the client is guilty of the crime (emphasis added).
If at any time before or during a defended trial a client makes a
clear confession of guilt to his or her defence lawyer, the lawyer may
continue to act only if the plea is changed to guilty OR the
(a) does not put forward a case inconsistent with the confession; and
(b) continues to put the prosecution to proof and, if appropriate,
asserts that the prosecution evidence is inadequate to justify a
verdict of guilty; and
(c) does not raise any matter that suggests the client has an
affirmative defence such as an alibi, but may proceed with a defence
based on a special case such as insanity, if such a course appears in
the lawyer’s professional opinion to be available.
The attorney can argue that the defendant should be released from custody prior to trial, and over what terms of pre-trial release are appropriate.
The attorney can advise the client not to do anything else that would make it easier for the prosecution to prove the client committed the crime (e.g. advising the client not to brag about having committed the crime on social media).
The attorney can advise the client on how to behave in the courtroom so as to avoid contempt of court sanctions, and how to dress and behave appropriately and in a manner that the court will find to be sympathetic (e.g. wearing a suit to court, acting respectfully, not speaking ill of the victim).
There are a variety of defenses not inconsistent with guilt that are available:
- One can argue that the statute of limitations has run.
- One can argue that the court that the case is being tried before does not have jurisdiction over the offense.
- One can argue that the crime was committed prior to the effective date of the law, or after it was repealed (if relevant).
- One can argue that the law in invalid under human rights treaties or similar grounds for invalidity.
- One can argue that the defendant has immunity from prosecution (e.g. due to diplomatic immunity, or because the defendant was acting pursuant to lawful military or court orders, or because of an agreement reached with a prosecutor previously perhaps in exchange for testimony in another case, or because the defendant acted based upon assurances from an authority upon whom he was entitled to rely that his actions would not be punished)
- One can argue that evidence is inadmissible for some legally recognized reason (e.g. lack of authentication or doctor-patient privilege or a marital privilege).
- One can argue that the facts presented by the prosecution don't establish the elements of the crime charged (e.g. no evidence has been presented that permission to use the property was not granted in a trespass case).
- One can argue that the facts presented don't establish the elements of the crime charged beyond a reasonable doubt even if some evidence is presented (but subject to limitations on what kinds of doubts may be suggested).
- One can make an insanity defense, or a lack of capacity to commit a crime defense.
- One can argue that a justification (e.g. self-defense) excuses the crime.
In the U.S., an attorney could implicitly or even expressly make a jury nullification argument, but I do not know if this would be permitted in New Zealand or not.
But one cannot argue, for example, that a different individual than your client committed the crime, or that the crime didn't happen at all.
This ethical limitation, by the way, is much more restrictive than the limitation under U.S. law, which prohibits an attorney from introducing evidence or testimony known to be false or fraudulent, but does not generally prohibit making arguments based upon possible inferences from the true facts that the attorney knows to be false inferences (i.e. the facts are true, but the conclusions that the lawyer asks the court to draw from the facts are not consistent with what really happened).
For example, a U.S. attorney could argue in a case with several co-defendants, that the victim might have been assaulted by one of the co-defendants rather than his client, based upon the evidence available, while a New Zealand attorney whose client had confessed to having assaulted the victim, couldn't ethically make that argument.
A defense lawyer in either the U.S. or New Zealand could find a forensic science article showing that the technique used to link the bullet to the gun by police investigators is unreliable and use it to cross-examine a prosecution forensic expert.
But, a New Zealand lawyer, unlike a U.S. lawyer, could probably not argue that there is alibi evidence putting your client on the other side of town at the time that the coroner erroneously believes the time of death to have occurred.
Of course, any counsel or advocacy provided by defense counsel in connection with a sentencing phase of the proceeding would also not be inconsistent with this ethical rule and, in practice, much of the value that a defense attorney adds to the process for a defendant comes at that stage of the case.