I don't know if this is much help Dee...I used to have a Meyer lemon tree in Melb and once when it did not get watered properly it dried up inside and went brown, but they also do that when they get frost bitten..It could be some kind of rot maybe the opposite from watering too much just a guess.

Does it get watered really well in the warm weather as they like a good drink and a feed regular I know you have had a lot of very hot days this summer so maybe it has missed out on occasion I do hope you find your answer. Cheers..MM.

Here's a link might be useful. You'll need Acrobat as its a pdf file.

Here is a link that might be useful: Lemon Diseases

Thankyou MM & Cosmicgardener for your help...much appreciated.
Cheers,
Dee.

it can also be fruit fly affected, they go brown inside as well, its very disheartening isn't it?

I agree......But there is no way it could be fruit fly as they dont have fruit fly in Melbourne.
And I hope the lazy fruit fly never gets up enough steam to fly over the great Murray River...MM.