We receive full age pension, and have been offered 3 month contract at $1000 each per fortnight. I realise we will have to declare this amount to Centrelink, but am unsure as to what happens with tax, should we get an ABN?

Hi Joy,
This depends on the company who is paying you. If they require an ABN to pay you as a contractor or whether they will pay you PAYG as employees.
If you are paid as a contractor you will need an ABN, in this case you will pay tax as a sole trader or partnership at the end of the tax year.
If you are paid as an employee, the company will pay you a wage plus super and also workcover and will tax you as though you are likely to earn $26,000 each for the year and you would claim back the excess in your tax return.
Check with the company first and then speak to an accountant if an ABN is required
Best of luck with the contract
Regards
Scott