For custom, I'd check out Oscar Hunt http://oscarhunt.tumblr.com/post/53322263776/custom-overcoats-for-winter. Think they will start at at least $850
For RTW, CT Outerwear is really pretty darn good for the price (look around for sales/coupon codes/cashbacks) http://www.ctshirts.com.au/mens-outerwear/mens-coats?q=auddefault|||||||||||||||
They have some slim fit options.
Or TM Lewin http://www.tmlewin.com.au/Coats/Mens-Coats-Coats,en_AU,sc.html
Or Brooks brothers