Originally Posted by
Gardyloo
Yes.
There's also a second problem, SEA-LAX is not on Oneworld metal (Alaska codeshare) thus not eligible for the Oneworld Explorer.
However your proposed route is less than 34,000 miles (around 31,500) so it would qualify for a Global Explorer 34,000 mile business class ticket (same price as a DONE4.) Alaska Airlines is now allowed for the GlobEx, so your route could fit within those product's rules. You might consult the Oneworld website about this.
Note too if you didn't want to transit LHR a second time AA flies nonstop BRU-JFK.
And if the OP really wants a DONE4 (for whatever reason) he could go SEA-DFW-LAX instead and then must utilize your suggestion of BRU-JFK to stay within the 16 segment limit.
The OP could also go LAX-MEL direct rather than LAX-SYD-MEL.