A party is not prevented from alleging dishonesty when costs are assessed, even if it was not raised before the trial judge. The principle established by Aaron v Shelton that a party who wishes to raise a matter of conduct in relation to costs must do so before the judge making the costs order was too broadly stated.
Conduct should be considered both by the judge making the costs order and then again during assessment. Ultimately the question is one of the proper construction of the order made by the judge. Where the conduct in question is dishonest, the judge should make clear whether he is making the order on the basis that at assessment the paying party will still be entitled to raise the dishonesty in arguing that the costs incurred in supporting the dishonesty were unreasonably incurred.
although Aaron v Shelton imposed too categoric a rule which meant that if you forgot to bring up conduct before the trial judge, you could not do so at the detailed assessment of costs, it is still important to ensure that issues of conduct (or failed or abandoned issues) are raised at trial. The judge may choose to reserve consideration of conduct until detailed assessment but if you fail to raise it at all, you may have difficulty in doing so for the first time at the assessment.