Thanks for your replies. There's a lot of repetitious info in the handbook which doesn't actually say much, other than the "TPMS illuminates a low tyre pressure telltale when one or more of your tyres is significantly under-inflated" and "if the tyre is over-inflated more than approximately 30kPa (4 psi),the horn beeps and the hazard indicators flash 3 times". There is no specific info about actual pressures, but the default pressure seems to be 35 psi, which is the recommended unloaded tyre pressure. Based on your experience, CamJam, the pressure can drop to about 25 psi before the low pressure warning activates, and you've gone as high as 44 psi with no warning.
The reason I'm chasing this up is that several weeks ago I destroyed a tyre on a slow, stony Pilbara station track and realised at that time that the TPMS system wasn't working. If it had been, I probably could have saved the tyre because it was a relatively small stone cut through the tread....one plug only. A couple of days ago, I discovered at a dealership that, yes, the system had been de-activated but could be re-activated with a Nissan Tyre Pressure Monitoring Registration Tool, which they did. But they did not know what the minimum and maximum pressures, below and above 35 psi, are before the warning activates. Anyhow, I'll do some experimenting with pressures, but I hope it's not going to be a bit of a pain in the neck.
Thanks again for your replies.