My electric hot water cylinder is on controlled power, I believe it turns on about 11pm daily.
For the last two days at about 5am, hot water has been pouring out from the standpipe, because the water in the cylinder has got so hot.

In order to stop the overflowing, I switched off the cylinder power switch and opened a hot tap for a few seconds.

I used a multi-meter to check a few things:  With the wires disconnected, the element is 35 ohms between terminals, and open circuit from each terminal to the metal cylinder casing or earth.  So apparently the element is fine.
The earth and neutral test as a good connection, via the MEN link at the switchboard.  So no broken earth or neutral wires, at least in the HW circuit.
Later, when the temperature was no longer crazy-hot, I also checked the thermostat, which is a T80 ST107.  It behaves as expected, open-circuit at a low setting, closed-circuit at a high setting.  Each thermostat terminal is open-circuit to the cylinder casing.  So apparently the thermostat is fine.

The low pressure cold water supply, after the nefa valve, enters via a tap underneath the cylinder.
The low pressure hot water outlet from the cylinder (to taps and standpipe) I expect is on top of the cylinder.  It's out of sight, within the ceiling space.

About a month ago, I replaced the nefa valve entirely, because the diaphragm had perished.  So I hope the nefa valve isn't the cause of the current issue.

YouTube tells me that Americans ought to flush their cylinders every few months (I think due to sediment and/or minerals and/or calcification).
I wonder if we have this issue and should follow this practice in NZ?
I've only been in my Whangarei home for 4 months.  Aside from the perished nefa diaphragm, I have no indication as to the age of the cylinder.
Even if my cylinder has sediment around the element or thermostat, I can't quite see how that might cause my issue.

At this stage, I'm thinking I should drain the cylinder, then replace both the element and the thermostat.
This may be tricky, since there isn't a dedicated 'drain' valve/tap (unless it's the garden-style tap located inside the garage).  I'll need to disconnect the pipe below the 3/4" brass tap, then connect my garden hose.

Does anyone have thoughts as to what the issue may be?