The only way to do it is to get **all** the Rollingstock construction lists from Railway Digest (Thanks M636C and your mates ) and the earlier ones in ARHS Bulletin and Railway Transportation etc
As M636C noted earlier, you then cross match that list with losses due to crashes, modifications and retirements.
EDIT; the Sept 2016 Railway Digest now at my local newsagent has one of these regular rollingstock construction lists so check it to see the high level of detail available. These lists are simply wonderful for loco, carriage and wagon researchers
Firstly, for anybody who hasn't checked it out:
But critically that list runs out about twenty years ago....
The current issue has the Rio Tinto article that I described earlier.
It is worth noting that the Contracts and Deliveries article is Number 160.
So at four per year, that completes forty years of reporting every quarter.
John Beckhaus was involved with every one of those articles.
The next article might say something about the forty years...
The tabulation in "Railway Transportation" appeared monthly, but it didn't start in the October 1951 issue..
Say it ran from January 1952 to March 1973 or so, that was just over 20 years.
In those days, the government railways published lists of acquisitions, disposals and conversions.
These days you can get a glossy brochure from Bradken but they won't tell you how many vehicles they supplied because of commercial confidentiality. Sometimes the placing of a big order will be reported including a number of vehicles but apart from the passenger vehicles where the government wants you to know how much they are spending you won't be told.
I regularly check the running numbers of Hunter Valley coal wagons, particularly any new order. Standing on the bridge at Carrington and counting vehicles works well, if you can see both high and low numbers. Photographing builder's plates where fitted is good, too.
At least one recent order had a real Chinese plate on the first car and not the bland "CIMC Australia" name and the year which tells you nothing.