Likewise I don't have trailer but have used Cook's Hire at Seaton (cnr Tapleys Hill Rd and Dumfries Ave - 8356 2036) for interstate round trips in the past.

Has been a fair few years now since I last needed one, but at that time they were one of the few places that were happy for their trailers to go interstate - their trailers seemed well maintained - good suspension/bearings/brakes - towed really well, so may be worth a call.