There is (or was) some conflicting information here, but I think we are getting to the root of it. Apparently MGA 1500 engines were imported completely assembled, and MGA 1600 engines were imported as complete parts kits entirely disassembled, to be assembled and run in (tested) after arrival.

On 9/14/2011, Thomas Aczel wrote:
    "The ex-Zetland guys still meet regularly at a pub for a yarn; perhaps there just might still be someone around with a memory stretching back over fifty years. I will try to follow up another couple of possible leads, including an acquaintance who did his apprenticeship at the Zetland plant (though I suspect his period there is much later than the MGA days). I’m also seeing John Lindsay in about ten days. He may just possibly be able to suggest somebody".
    "PS Just got off the phone from John Fogarty, BMC Fitter and Turner from 1959, and owner of two MGAs (Coupe and Roadster). He’d thought the MGA engines were imported assembled!! He’s going to check with a chap called Tony Dingle tomorrow who may be able to give us a reliable answer".

On 9/15/2011, Thomas Aczel wrote:
"I’ve just heard back from Tony Dingle via John Fogarty. Both these guys, (along with Peter Fogarty, John F’s twin brother) worked at BMC Zetland from the the '50s. Peter says that “no way can he imagine that the MGA engines were locally assembled from imported components”. Peter and John Fogarty and Tony Dingle ALL feel the MGA engines and gearboxes were fully imported, assembled. He (and John Fogarty) clearly recall a large cage just past the theatrette where the 140 odd apprentices would go on Fridays for presentations containing the built up MG engines, complete with carburetors and gearboxes, all bolted together. (Apparently there was a problem with the SUs getting pilfered from these engines). Anyway, John Fogarty intends to take this up with John Lindsay to clarify why John L thinks otherwise, but it does indeed seem more likely that the MGA engines were indeed fully imported, assembled".

On 9/16/2011, Thomas Aczel wrote:
"I’ve just heard back from Tony Dingle, via John Fogarty. Tony Dingle’s spent quite a bit of time ringing around ex-BMC (Australia) guys today. It appears that the 1500 MGA engines were fully imported. The 1588 and 1622 engines were imported partly assembled as “short engines”, with pistons and con-rods fitted, but not the heads or crankshafts. The main bearings were locally sourced, we’re not certain, but suspect from Repco. The engines were assembled from this form to increase the local content for the purposes of obtaining import duty concessions".

It seems unreasonable that pistons and conrods would be installed without the crankshaft prior to shipment. Perhaps it means the pistons were selected to be size matched to the bores, and shipped along with the engine blocks. There are also alternate stories on the main bearings being locally sourced, perhaps only for some "specials".

On 3/22/2012 Tony Dingle in Coffs Harbour, New South Wales, AU wrote:
"Despite some of the comments made, my info relating to the local assembly of the various MGA engines was gleaned from the production supervisor for the assembly section". -- Kind Regards - Tony Dingle

On 9/16/2011, Thomas Aczel wrote:
"Incidentally I’d previously been told that BMC (Australia) had designed a modification to the rear main bearing of the earlier B-series engine to reduce the inherent leak from here, prior to the incorporation of a proper seal in the later engines. They (including Peter and John Fogarty) were fitting these modified bearings at BMC to returned cars, four cars at a time on the large factory hoists. The demand became so great that Repco began supplying the modified bearing design, and they were at the time available “off the shelf”. Peter Fogarty incidentally, a very skilled engineer, still will modify these rearmost bearings and shells on a one off basis for folk. I can personally vouch for the effectiveness of the modified bearing, as it’s eliminated 90% of the leak I once suffered at this point. One of my better $30 investments".

The following off-site link provides a page discussing MGA engine assembly at Zetland (among other things). https://www.mgccwa.com/Marque%20History.htm
Do consider that there may be some discrepancies in this article, such as all MGA engines being assembled as Zetland. It is now fairly sure that MGA 1500 engines were imported completely assembled, while MGA 1600 engines were imported as complete parts sets to be assembled at Zetland.

At 01:46 PM 9/27/2011 +1000, Thomas Aczel wrote:
I had an interesting few hours with John Lindsay last Friday. John retains vast amounts of old BMC (Australia) material, much of which he's yet to scan and store in an ordered fashion. John remains absolutely adamant that the MGA engines were fully assembled in Zetland from imported components. (not "partly assembled" as we were told. He can even name a couple of the assemblers!)

The truck that carried the pallets of MG and other assemblies and parts
from the BMC plant in Zetland to the Pressed Metal factory.

On 10/8/2011, Thomas Aczel wrote:
"I just got off the phone from John Lindsay. I realize this will be considered controversial (however, mainly by those who weren’t there at the time), but John is adamant that all MGA engines for locally assembled cars, were assembled in Australia. The components were all fully imported (which he thought odd at the time when many of the items were manufactured here). John says that the local engines were plain metal castings or painted gray as they went down the line. The MGA engines (which were assembled in batches) were however painted red and stood out very obviously. John clearly remembers the excitement when the first MGA engines were hot run tested, as everyone was eager to see how the twin carburettor MG engine performed in comparison to the local 1500 (single carb) engines.
John was less categorical about the gearboxes, not remembering with certainty, but thinks it is probable as “assembling gearboxes was dead easy”, and all the necessary tools were there on hand, and BMC were eager to get the local content up any way they could to reduce import duty rates.
John also added that he thought Allan (“Roger”) Foy’s talk (CKD-AUS on your MGA Guru site site) is a very good summary on how things were organized at the time.
John is going to try to find out for me about the source of the suspension springs, as he has some access to the microfiche files from that period through Rover. (I thought they were defunct. Perhaps he was referring to the local club). He recently found the MGA oil cooler there for me, but the quality unfortunately was too poor to reproduce".

On 2/1/2012, Thomas Aczel wrote:
"John Lindsay was able to find out late last year, through a former BMC work colleague, that the MGA 1500 engines were locally assembled and that the 1600 and 1600 MkII engines were fully imported (like the subsequent MGB engines)".