It has been long established (hundreds of years) that if you say the higher rate of interest is the official rate and the lower rate is a concessionary rate, then the clause will not be struck down as a penalty (whereas the other way around and it will be).
This old rule was once criticised by an old fool. Lord Eldon in 1802, said it favoured form over substance (when equity as a rule is suppose to favour substance over form). The borrowers sought to make something of this and defeat the rule.
The borrowers were probably encouraged by the fact that the trial judge could not find any appellant court which had expressly approved of the rule. The court of appeal on the other hand did:
In David Securities Pty Ltd v Commonwealth Bank of Australia (1990) 23 FCR 1 at 29 Lockhart, Beaumont and Gummow JJ held that it was well established that a covenant offering a reduction in the rate for prompt payment does not attract the penalty doctrine. One of the decisions cited by their Honours for that proposition was Brett v Barr Smith  HCA 4; (1919) 26 CLR 87 at 94, where Isaacs J referred with approval to Lord Hardwicke’s judgment in Nicholls v Maynard 3 Atkyns’ Chancery Reports 519 at 520.
Despite this, the Court of Appeal agreed there was an arguable case at least in the high court that the old rule should be overthrown.
However, this concession did not win the day for the borrowers. The Court of Appeal found that nonetheless it would be against the balance of convenience to allow the appeal.
This was because even if a stay were ordered and the appeal was successful, the borrowers would still be required to pay the amount owed and the costs of funds on that amount to the lender. The court found that the risk that granting a stay would reduce the amount recoverable to the lenders outweighed the disadvantage to the borrowers. The borrowers were not in a position to offer any security to protect the lender against this risk.
The court refused the stay.