Complex sentence: They were taken aboard the "Lady Castlereagh" which around mid-December 1817 set sail for Australia, arriving at Port Jackson on 30 April 1818.
Split into simpler sentences: They were taken aboard the "Lady Castlereagh" which around mid-December 1817 set sail for Australia. The "Lady Castlereagh" arrived at Port Jackson on 30 April 1818.