I have been there, which Island are you thinking of going to?.
Oahu is a very busy Island, the other islands are much quieter in comparison.
You would have to break the journey (its not possible to do this in just one day as the flight connections would not fit) and the flight duration from the West Coast to the Islands is a further 6 hours. Also AA and Hawaiian airlines (you cannot book direct with Hawaiian but would need to use their sole UK agent instead) would charge you bag fees for the second and any other suitcases.
IIRC you can get a direct flight to Kahului airport on Maui from one of the West Coast US airports on either AA or Hawaiian airlines. You could also try booking with American Airlines as UK customers CANNOT make bookings via Hawaiian Airlines website.
Maui roads are single lane highways so driving to Kapalua will be okay particularly in daylight hours.
All in all it will involve two flights; one to the West Coast (that's about 10 hours) and a second one to the Islands (that's around 6 hours). Not quite as far as Sydney but its still a long way. You cannot do this journey in a day as the flight connections will not fit; there will need to be an overnight stopover on the West Coast before flying on the next morning.
It is possible to book flights through the Hawaiin Airlines website if you live in the UK. Here's how...
Book in the usual way In the payment section choose USA as country of residence Enter your UK address and city/town Select a US state and zip code (we used ones of a US relative, but any would do) Enter your credit card details as usual
Make a note of the booking reference and a day later check in Manage Bookings and the flightvshoul be confirmed.
You can double check with your CC company to make sure the payment has gone through.
We have successfully booked return flights to Maui this year and last through their site. Once you have an account with them i think it gets easier. You even get to collect miles with them.
Dont know why Attila wouldn't recommend my suggestion. I am a UK resident. As i said we have booked flights to maui twice using this method with no problems. You have nothing to lose by giving it a try, you will be saving pounds if it works.
I initially found out about this method when we had the same problem. A quick search on google found the solution i posted.