The Best way to handle this is to send them a 'letter of demand' outlining your concerns and how you wish the contract should be handled.
For instance, you should outline that you want the work to be completed as agreed upon, even attaching documentation of the original agreement (if documentation exists) a give them a period of time to get back to you in relation to work completed.
Aslo conclude in the letter that should they not reply in (No of days - for example 21 days) you will seek legal asistance to enforce the contract upon them or sue them for damages.
The delay of over a year is extremely unreasonable and should be dealt with as soon as possible.
The contract amount is large and therefore should legal action be taken, it should be dealt with in the Supreme Court of New South Wales. A Statement of Claim should be filed - that is a document filed with the court and served upon the other party to allow you to sue them. This should be acompanyed by a statutory declaration and a copy of any such agreement being made.
This will set the matter in court if they file a defence against the claim. Should they not file a defence within 28 days, you can file for a default judgement and then be able to recoup the money. Should they have a defence, the matter is listed for a hearing.
Any Solicitor in Sydney should be able to assist.
Before any action be taken, mediation - where both parties can negotiate should always be an option, however due to the other side taking a long time, it may not be a suitable option.