Are oral law a legal contract in austalia?

if buyer were to buy a notebook for $1k after seeing the advert for sales in the newspaper. seller n buyer agree for payment in 4 installment. first two installment was in time while 3rd installment was delayed. buyer contacted seller that he would delay payment n seller agreed to the new date for the 3rd installment. later the seller send a letter to buyer that he do not want to sell the notebook. wat can the buyer do since all this are done orally?