How high can I build a retaining wall before

How high can I build a retaining wall before I need council approvall

I plan to build one 500 mm high and 7 metres long out of treated pine

200 x 72 mm trreated pine with 175 x 75 mm posts in the ground

That's probably a question for your council.

FWIW I seem to have a maximum height of 1 metre in the back of my mind, not sure if it's correct though.

Give them a ring. But I think you'll be OK.

I checked a fwe weeks back in my area. Melbourne vic..South East suburbs (Not sure if that matterS)
It was 1 meter.

Yeah that seems to be about right silent. I did some retaining work on some properties I had in Sydney and I am sure thats what it is.
But if you need to go higher you can tier the retaining levels without council approval.just have to keep each tier under the required height.

Scone Shire Council-600mm
Muswellbrook Shire Council-900mm

These 2 shires are nextdoor to each other!

Don't assume anything and speak to your local council regarding compliance.

Most councils are high than 1m, but there are also considerations for retaining walls positioned right on property boundaries

Willoughby City Council (Sydney Nth Shore) 300mm :o:o:o:o without a DA, fees etc., even if just replacing an existing wall that's fallen down.

There have been some recent decisions made by the NSW courts about Councils making unreasonable impositions in private landholders - the Councils lost both the case & costs in the latest one.