The critical question is whether the rejection of the offer was unreasonable in the circumstances. We see no justification for a more stringent test such as "manifestly" or "plainly" unreasonable.
Of course, deciding whether conduct is "reasonable" or "unreasonable" will always involve matters of judgment and impression. These are questions about which different judges might properly arrive at different conclusions. As Gleeson CJ said recently, "unreasonableness is a protean concept". But a test of reasonableness is, we think, entirely appropriate to the exercise of a discretion such as this.
Factors relevant to assessing reasonableness
The discretion with respect to costs must, like every other discretion, be exercised taking into account all relevant considerations and ignoring all irrelevant considerations. It is neither possible nor desirable to give an exhaustive list of relevant circumstances. At the same time, a court considering a submission that the rejection of a Calderbank offer was unreasonable should ordinarily have regard at least to the following matters:
(a) the stage of the proceeding at which the offer was received;
(b) the time allowed to the offeree to consider the offer;
(c) the extent of the compromise offered;
(d) the offeree's prospects of success, assessed as at the date of the offer;
(e) the clarity with which the terms of the offer were expressed;
(f) whether the offer foreshadowed an application for an indemnity costs in the event of the offeree's rejecting it.
It has been argued on occasion that the maker of a Calderbank offer should not be entitled to costs unless the offer sets out, with some reasonable specificity, the basis for the offeror's contention that the offeree should accept the compromise - for example, because the offeree's case was hopeless or because the offeree had no reasonable prospects of doing better in the proceeding than was being offered in advance.
Once again, we think it neither necessary nor desirable to lay down any general rule in this regard. We agree with what Redlich J said in OCBC, as follows: