Unfortunately October is one time of the year when it is not easy at all to suggest such a place. Both the Gulf side ( eg Koh Tao excellent for diving August and September ) is not good in October, Phi Phi / Phuket side September and October are heaviest rainfall months of the year ).
If you could make it ‘very early’ September what you can do is Koh tao for diving and then just ‘island hop’ to koh Phangan or to Koh samui ( lively and busy but so busy as it would have past it’s August and september busy season ).
To put some of that into perspective average rainfall over past years in mm per month
September:
Koh Samui and Koh Tao area : 113 mm
Phuket and Phi Phi area: 417 mm
October:
Koh Samui and Koh Tao area : 265 mm
Phuket and Phi Phi area: 350 mm
It is the time whren the seasons change and can be very changeable.
The good news – accommodation rates pretty much everywhere are very low in October. If you can amange to retain flexibility and ‘wing it’ that gives you the chance to check rainfall for coming days once in Thailand. Spot where is fine and 10 minutes in an Internet cafe can secure travel and accommodation online easily.
You ‘can’ affod to wing-it as the expression goes in October for both domestic travel ‘and’ accommodation as very quiet time. Weather varies so much in october that one time of the year when it is a good idea to leave your options wide open.
From perth you can fly via SIN or KL into Phuket or Bangkok as you wish and then ‘take it from there’. deceide your first place and book say 2 nights so you you know where you are going and then check the local weather ‘local’ to see what is going on.