Bear in mind that voluntary promises are not enforeceable and pleadings may often be amended. The breaking of a promise without more may be morally reprehensible, but not unconscionable - something more than a broken promise is needed. Here, to hold the Commonwealth to representations which would deprive it of defences which were available to it by statute or general law, would be a disproportionate response. An order for costs may be sufficient to meet prejudice in terms of expense and inconvenience occasioned by the need for pleading new defences. Brennan J Election estoppel and waiver are related ideas.
Election involves a choice between rights.
Here we have a unilateral release or abandonment of rights - waiver in dealing with abandoned rights does not require us to consider whether another party has acted in reliance on the release. Estoppel ensures that where someone has acted in reliance on what another has promised, they should suffer no unjust detriment. Each doctrine has a different purpose. Waiver can only be in respect of a right which is introduced solely for that person's benefit. It can only arise when the time for exercising it has arrived - a mere intention not to exercise is will not do.