Give Lumley Specialised Vehicles (LSV) a call. I've had my skyline insured with them since i bought it, before i turned 25. They were really helpful for me in trying to get the best (cheapest) possible premium. My excess is only $500 (+ age when i was under 25) and i get my choice of repairer. After watching my bf go through hell with AAMI (who underwrites Just Car), with their so called "recommended/guaranteed repairer", that is very important to me now. When i joined they were called Unique Car and were then Vigil Underwriting. I've also got my daily driver and my bf's car insured under the same joint policy to get multi vehicle discount. They do not have pay by the month, but they do quarterly payments for no extra cost.
Their number is 1800 651 014, but they're only open in business hours.
(no i do not work for them, but they have been the most helpful insurer so far, so i'm more than happy to recommend them).