Designing a PoE Circuit

I'm going to buy a PoE injector kit, and I will supply it with an AC voltage, and with ethernet cable, and at the other end I will design a seperator circuit, but however most PoE injectors just give 40DC, with at most 400mA. I would like to get more amperage at the backend, is it a must to have a battery or any other supplier and put some transistors to get more current ?

I'm not really sure what you're asking, but the 40V @ 400mA (16 watts) can be stepped down to lower voltages and higher currents if desired. e.g. 12V @ < 1.3A

I will have a PoE PSE coming from an adaptor, then I want to design a PD circuit. I want the PD circuit to deliver ethernet, and multiple voltages like 12(1A), 3.3(2A), 5(4A). Is that posssible ?

According to https://en.wikipedia.org/wiki/Power_over_Ethernet the max power available at the PD is 25.5W. If this is sufficient, then you can use a SMPS to provide the voltages you require.

Note: 12(1A), 3.3(2A), 5(4A) totals 38.6W, so it's not supported by 802.3at type 2. Apparently the non-standard configuration can provide more power... though it's non standard and may be harder to source the PSE..

How do you like this ?

https://www.electro-tech-online.com/custompdfs/2013/01/dn425f.pdf