Motherboard is showing 24v instead of 12v

I have put together a new PC build and when I tu on the machine it's showing me the following error/waing:
"Asus anti-surge was triggered to protect system from unstable power supply"

Besides that, everything functions as normal.(though I couldn't get windows to boot from the SSD after installation, but I'm assuming that's a different issue)

In the bios the 5V and 12V are both highlighted in red. The 5V is showing as 6.440V, and the 12V is showing as 24.480V.

I have the 20+4 plugged into the main motherboard socket, and one of the CPU4+4 plugged into the other 4-pin socket.(it comes with 2x separate cables labelled "Left" and "Right")

I haven't been able to procure a DMM yet so I'm not able to definitively confirm if it's the PSU. But from looking online I get the impression it's more likely to be the motherboard.

I've tried resetting the CMOS settings in bios and removing the motherboard battery.

My equipment is:
- Motherboard - B150M-A: https://www.asus.com/au/Motherboards/B150M-A/overview/
- PSU - RX-535AP: http://www.raidmax.com/power-supplies_rx-535ap.html

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