I can't speak for Queensland, but in NSW the ute is a commercial vehicle & is designed to cary a load that can quite lawfully be stacked high enough to completely obscure the internal [central] mirror, so all vehicles that are registered in the commercial category [Utes, Vans & Station Wagons] are required to have two external mirrors, with the left side one offering a view to the rear on the left when the internal mirror is obstructed.

So lets look at the numbers. Tyre diameters.
185/60/13 .................... 552mm [my preference for fronts]
600x12- 4 ply ................ 574mm Highway tread, Ute front.
600x12- 6 ply ................ 574mm Highway tread. Ute rear.
155R12- 4 ply 'rating'....... 554mm Highway tread.
155R12- 6 ply 'rating' ...... 554mm Highway tread.
Bonus numbers.
195/60/13 .................... 564mm
205/60/13 .................... 576mm [My preference for rears]

The 185/60/13 is about 0.45% smaller than the stock radial.
The 205/60/13 is about 0.4% larger than the stock cross ply.

All tyre pressures are 'cold inflation pressure' The maximum load ratings for the front tyres are...
4 ply 600x12 cross ply, ............ 385kg @ 220kpa [32psi]
4 ply 'rated' 155r12 radial ........ 395kg @ 220kpa [32psi]
185/60/13 radial ..................... 450kg @ 240kpa [35psi]

The maximum load rating for the rear tyres are ......
6 ply 600x12 cross ply, ............. 410kg @ 250kpa [36psi]
6 ply 'rated' 155r12 radial .......... 420kg @ 250kpa [36psi]
185/60/13 radial ...................... 450kg @ 240kpa [35psi]
Bonus numbers
195/60/13 .............................. 487kg @ 240kpa [35psi]
205/60/13 .............................. 530kg @ 240kpa [35psi]

Speed rating codes. This is the maximum "sustained" speed that the tyre is rated at when the tyre pressure is at or above the minimum required to support the load imposed upon it.

J=100kph
K=110kph
L=120kph
M=130kph
N=140kph
P=150kph
Q=160kph

Have a real good look at the data on the sidewall of your tyres as the actual data may be at variance with my book, but simply put, if the load rating is equal to or greater than the originals, then that removes that grounds for rejection of a safety slip.

The original wheels were 4" wide & the rear track is 49" [1245mm?]
A 1" increase in rim diameter, particularly since the tyre size doesn't go below 60 series shouldn't ruffle any rego departmental feathers.

Tyre Data Source
The Tyre & Rim Standards Association Standards Manual 1988.
I know it's 20 years old but the only changes would be in the 185/60/13 & if any changes exist, it would be to uprate the specs.

Datsun Data Source.
1972 Datsun 1200 Passenger model sales brochure.

Knowledge is power, so hopefully this will give you the power to succeed.