And here's some more detail
1An offer
The contract variation will start with an offer by one party. The offer must be for something in exchange for something such as a product or service. The verbal or written variation to the agreement must be on proposed terms (e.g. the product/service being offered and the price to be paid). An example is a verbal offer over the phone for a contract variation to a mobile phone agreement to move onto a higher monthly plan.
Circumstances to watch out for
the varied terms of the offer are lawful
Some contractural variations are not allowed by the law, such as replacing the names of the parties.
offer was rejected
If an offer was rejected by the party it was directed to, then it ceases to be an offer.
offer was withdrawn
If the party who made the offer then withdraws it, then it ceases to be an offer provided that it was not already accepted by the party the offer was directed to.
expiry of offer
If the offer expires, then it ceases to be an offer. If the varied terms do not include a specific discussion on how long the offer is open for, the offer expires at what would be a "reasonable" time for that kind of product or service.
there was a counter-offer
If an offer was not accepted, but the person who the offer was directed to made a counter-offer, then the orginal offer ceases to be an offer.
uncertain or incomplete of terms
If important or "essential" terms in an agreement are uncertain, incomplete, vague or meaningless; and if additional evidence cannot establish what the term was meant to mean, then there cannot be a valid offer.
commercial offers over $40,000
The Australian Consumer Law will imply terms (e.g. guarantees that products are fit for their intended purpose) into agreements unless it is a business related transaction where the value of the product or service is over $40,000.
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2Acceptance
Acceptance by the party who the offer to vary the contract is directed to. This must be to the other party to the original agreement. An acceptance shows a willingness by that party to accept the offer to vary the agreement without further negotiation, and be bound by the proposed terms in the offer.
Circumstances to watch out for
acceptance must be communicated
An offer to vary an agreement cannot be considered accepted if the acceptance was not communicated to the party that made the offer.
a term that outlines a procedure to vary
If the original agreement is in writing, there may be a term in the orginal agreement that outlines how the agreement can be varied. Such a term may specify how an offer to vary an agreement can be accepted (e.g. in writing or by post). This will depend on the formality of the original agreement. In the case of more informal verbal/handshake agreements, if the parties discussed rules on how their agreement could be varied in negotiations of their original agreement, then these rules would apply.
proof of acceptance
There is rarely an issue proof of acceptance of a variation to an agreement in writing. While a verbal acceptance is a valid acceptance, it is wise to have a witness present, or to send an email confirming acceptance so that there is proof establishing the variation of the agreement should an issue occur at a later date. A verbal acceptance followed by both parties performing their obligations under a verbal agreement of the contract variation may also be sufficient to establish proof of acceptance.
unfair pressure
There is no valid acceptance if one party unfairly pressures the other party into accepting a variation to an agreement.
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3Consideration
Something of value must be exchanged for what was offered in the variation. It could be something of value given or promised such as money, a service or even a peppercorn.
Circumstances to watch out for
no additional consideration provided
Any variation to an agreement must have fresh or additional consideration exchanged by the parties. This means that the value exchanged in the original agreement will not be viewed as consideration for the contract variation to be enforceable.
adequacy of consideration
Any consideration can be good consideration, therefore it need not be adequate consideration. This means that a will be valid even if one party to the agreement does a bad deal and offers greater value than what is exchanged in return.
the variation being a pre-existing duty
A promise by one party that they were already bound to do under the terms of the agreement is not viewed as adequate consideration.
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4Intention to create legal relations
This is to ensure that both parties wanted to be legally bound by the variation to the agreement. Being legally bound means that the agreement is enforceable in a court. Compare this with being morally bound which is not enforceable in a court, no matter how much one party feels hard done by by the other party.
Circumstances to watch out for
where no consideration provided
If there was no consideration exchanged then it may stand to reason that the parties did not intend to be legally bound by the agreement.
commercial agreements
In business to business or commercial agreements, it is usually presumed that the parties are intended to be legally bound.