I think I can shed quite a lot of light on your gg grandfather Isaac Maskell as he was my 3x great Uncle !!His brother Abraham was my 3x great grandfather.
I have only recently found out about an Australian connection though ........ and as far as I can see Isaac probably went to Australia but didn't stay there as he had his family in Brightlingsea and appears living there in the 1841 census (age 25, Mariner), 1881 census (age 70, Mariner) and 1891 census (aged 81, widowed) - ( I haven't got 1851, 1861, 1871 info yet).
Isaac married Elizabeth Holden on 15th Jan 1836 in Brightlingsea and there are records of them having at least nine children in Brightlingsea between 1836 and 1858.
I need more time to tell you what I know about Abraham (who maybe went to Australia with Isaac) because if I rush I may get my facts wrong ... there were quite a few Abraham and Isaac Maskells.
If you give me your e-mail address I can send more info as attachments over the next few days.
My great grandmother (mother's mother's side) was Maud Mary Maskell, daughter of John Cousins Maskell who was the son of Abraham Maskell and Mary Cousins - this Abraham being the brother of your 2x great grandfather Isaac.