Welcome to the Bunnings Workshop community. Thank you for joining us and sharing your question about how much weight can the Montgomery shelves hold.
The shelves on the Montgomery 6 Tier Chrome Wire Metal Shelving Unit With Wheels are actually very well made and can take a fair bit of weight. I would hazard a guess that its maximum weight capacity per shelf is around 30kg evenly spread. It isn't actually the shelves that would fail but the plastic stoping inserts would deform and force their way out through the top.
One giveaway that you have placed too much weight on the shelf is that it starts going lower as each day passes by. If you are after a heavy-duty shelf, I recommend the Montgomery 1830 x 910 x 410mm 5 Tier Shelving Unit with a rated capacity of 150kg per shelf. Would it be possible for you to tell us what you're storing on the shelves? This will allow our members to give you a comprehensive recommendation on what shelf to use.
If you need more advice or information, please let us know.
I see my colleague @EricL has been assisting you. I've been able to acquire some additional information on this unit's weight capacity, which adds to Eric's conservative estimate.
With the levelling feet installed and weight evenly distributed, each shelf can hold up to 250kgs. With the castor's installed and the weight evenly distributed, each shelf can hold up to 60kgs.
Please let us know if you have any additional questions or need further assistance.