I was born in Australia but left when I was 20.I have been living in Greece for 31 years now,but I plan to return to Australia permanently.Have I lost any citizen rights?Am I entitled to a pension and at what age?

I suggest you ask at the Australian consulate what your status would be - that is, whether you would be considered a citizen in Australia or would require a permanent residence visa. Regarding your pension question - I found a web site that indicates there is a 10-year residency requrement to receive an age pension in Australia - see Age Pension - residence requirements (http://www.centrelink.gov.au/internet/internet.nsf/payments/age_residence.htm)

It states:

To be paid Age Pension, you also need to meet the 10-year qualifying Australian residence requirements, unless:

you are claiming under an international social security agreement, or
you are a refugee or former refugee, or
you were getting Partner Allowance, Widow Allowance or Widow B Pension immediately before turning Age Pension age, or
you are a woman whose partner died while you were both Australian residents and you had 2 years residency immediately before claiming Age Pension.
Note: The 10-year Australian resident requirement means you have been an Australian resident for a continuous period of at least 10 years, or for a number of periods which total more than 10 years, with one of the periods being at least 5 years.

So next step is to see whether Australia and Greece and have any sort of social security agreement that would allow credits for pension you've earned in Greece to be transferred to Australia. Again, the consulate should be able to answer that for you.

I was born in 1956 in the UK and successfully migrated to Australia under the skilled workers visa application. I lived and worked in Adelaide from 1988 - 1994 and became an Australian citizen. I have lived in the UK since 1994 but intend to return in 2011 to open up an office for my current employer.
I will have my wife (also an Australian citizen) two children 13 & 11 years old (citizens by descent) and my elderley mother-in-law.

If I work until I am 65 will I be eligible for an Australian state pension?