Looking around on the net, there are many plans for making an AM radio transmitter.

One in particular that caught my eye was this low cost project -

http://sci-toys.com/scitoys/scitoys/radio/am_transmitter.html.

Assuming I can source the parts needed, just wondering if such a project will work with an Australian radio?

Hi and welcome to the forum. This transmits in the middle of the broadcast band, so, yes, it is ok for your Little Nipper.
This is a very basic device and I suggest trying to use it could be a pretty frustrating exercise.
I would suggest one of the many designs with a range up to a few tens of metres at least, a means of setting the audio input level and some way of setting the transmit frequency away from any local station.
When set up, you can enjoy music or programs of the period of the radio, very satisfying.

Great. Thanks for the reply!

That's an interesting design,
but I can't see if the crystal is fully powered most of the time,
and the coupling interrupts the normal flow of DC current from the battery,
or if the DC from the battery is only a circuit completed by the
transformer secondary when excited by audio on the primary.

Is the blank carrier the strongest AM signal that is subtracted from by audio modulation,
or does the modulation of the carrier add to it's power?

AM modulators are pretty basic in concept, unlike things like television or FM stereo or digital TV, the laws of physics dictate the standard. Which means any and every AM transmitter can be heard on all and any type of AM radio.

I built a somewhat more complex circuit similar to your reference above:-

The crystal oscillator module isn't directly modulated, but a subsequent RF buffer is. This would allow deeper modulation depth, without the oscillator cutting out because of a lack of supply voltage.

Here I used a higher frequency oscillator module, then divided it down with a TTL counter chip to somewheres in the MW AM band. Be sure to select a frequency not occupied by a strong, or moderate strength, or even a weak station, to avoid hetrodyne whistles from your carrier and a station's carrier interacting with each other.

Or if you want to use tubes in a small AM transmitter

So it is that way then, I've had it upside down my whole life.

So a radio station would use more power when transmitting dead air, than when actually speaking or playing music.

Wouldn’t it be a quality problem with both of them that the oscillator is a TTL can
producing a square wave at varying levels rather than a sine wave?

As far as I can tell, the carrier would be a square wave, and only the modulated audio, a sine.

Some time ago I built a crystal locked fixed Signal Generator. I used a Quad NAND gate two for the oscillator & they passed the signal to a third. That, and the audio were fed separately to a dual gate FET, in a buffer configuration. Than allowed the modulation level to be varied.

It was discovered some time ago that if you fed in a frequency that would not interfere & could be filtered out. That would enable the Valve Finals (& anything else) to be maintained at a more constant current.

Marc