The Judicial College has recently updated their Criminal Proceedings Manual to take into account the observations of the Court of Appeal in Buchwald v R  VSCA 445 and Waters v The Queen  VSCA 415, decided just before Christmas last year. It can be found at 126.96.36.199 - Judicial Intervention.
In Buchwald, Hansen JA described questioning of the accused by the trial judge [at 121]:
Counsel for the applicant did not identify particular questions that were objectionable. Rather, he submitted, the vice was the cumulative effect of the questions – in essence, that the judge expressed scepticism, incredulity, highlighted inconsistencies and effectively accused the applicant of recent invention. In combination with the prosecutor’s allegations of recent invention (to which we refer below under ground 2A), the judge’s questioning must have significantly undermined the applicant’s credibility. That was demonstrated, submitted counsel, by the fact that the jury specifically asked to see transcript of the judge’s questions, and then returned a guilty verdict merely 15 minutes later.
Some extracts of the judge's questions are reproduced in the judgment. The Court of Appeal held that the judge did not cross the line and lose his impartiality, despite asking some probing questions.
In Waters, it wasn't the accused but the prosecutor who was cross-examined by the trial judge.
Ashley JA [at 93}:
How did the way in which the Crown finally deployed the evidence against the applicant come about? It must be recognised that it was for the prosecutor to decide how the case should be conducted. The judge could not compel the prosecutor to present the Crown case in a particular way. On the other hand, it was the judge’s intervention, on multiple occasions, which introduced the idea of the Crown ‘refining’ its case, which suggested (albeit that it came to nothing) a different way of looking at the evidence referred to ... above. This involved cross-examination of the prosecutor about the evidentiary possibilities of calls 1888 and 775 – and this in turn led to the prosecutor’s ultimate concession as to the force of his Honour’s persistent analysis – and which, I infer, encouraged the prosecutor thereafter to take the bit between his teeth and further ‘refine’ the Crown case.
Both cases make for interesting reading, as does the summary of relevant principles in the revised Criminal Proceedings Manual.
What none of the materials do, of course, is to provide any definitive guidance on what advocates should do when a judicial officer is seen to be intervening more than it's thought they should ...