G'day Steve,
When I was in NZ April 2013 I was in Dunedin and saw and photographed the Rangi which I thought looked like she was built off the same drawings as the Iron Cove. Iron Cove was built in South Australia in 1963/4 and if I remember correctly I photographed her on the slips at Sydney Slipways in 64, before she went into service with Fenwicks.
I built a model of her in 1983 and she's still in good order all these years on. At the time, I was told that no drawings existed due to a fire at the builders yard, but I was able to get a GA which I used along with my old B&W photos, but had to cheat with the hull by using a F/glass hull of a pommie tug. The Iron Cove has a Hydroconic hull design which is a bit ( lot ) different to the hull I used, but if I don't tell anybody, they don't know! The scale I used works out at about 9/16":ft which was necessitated by the hull length for a model of 4'10 1/2". She runs a 6"x 4 blade prop. It was all scaled off the GA using a proportional divider.
If this is what you are looking for, I could probably find the GA and get a copy done.
Regards,
Ian.