The best approach is to iterate over the hindcastYears.

For each hindcastYear iterate over all the available hindcastMonths and for each hindcastMonth iterate over all the available hindcastDays.

At this point you may wish to check BoM availability and to view a BoM request

for hindcastYear in hindcastYears
    for hindcastMonth in hindcastMonths        
        for hindcastDay in hindcastDays             
            hindcastDate = hindcastYear-hindcastMonth-hindcastDay               
            S2S-request(hindcastDate)

• The objective of this example is to demonstrate how to iterate efficiently over all the available hindcastYears, hindcastMonths and hindcastDays for a BoM reforecast request
• It can be used as a starting point, however you need to keep in mind that you have to adapt it to your needs eg to set the keyword values according to your requirements ("param", "levtype", "step" etc).
• In this way you can extend this request to download the whole S2S BoM reforecast.

 Please note:

• the most efficient way is to request all hindcastDates of a hindcastMonth, in one request, like the example below.
• you can use the variable target to write the requested data as you wish. In the example below the data is written per leveltype (sfc, pl) per hindcastMonth.
  

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()

origin = "ammc"
modelVersionDate = "2014-01-01"

def retrieve_BoM_reforecast():
    """
       A function to demonstrate how to iterate efficiently over all hindcastYears, hindcastMonths etc
       for a particular BoM_reforecast_request.
       Change the variables below to adapt the iteration to your needs
    """
    hindcastYearStart = 1981
    hindcastYearEnd = 2013
    hindcastMonthStart = 1
    hindcastMonthEnd = 12
    # BoM availability is every 5 days: 1, 6, 11, 16, 21, 26
    hindcastDays = [1, 6, 11, 16, 21, 26]

    #Step 1: Iterate over all the available hindcastYear(s)
    for hindcastYear in list(range(hindcastYearStart, hindcastYearEnd + 1)):
        #Step 2: Iterate over all the available hindcastMonths(s)       
        for hindcastMonth in list(range(hindcastMonthStart, hindcastMonthEnd + 1)):
            hindcastDates = []
            #Step 3: Create the list of the available hindcastDates
            for hindcastDay in hindcastDays:
                hindcastDate = '%04d%02d%02d' % (
                    hindcastYear, hindcastMonth, hindcastDay)
                hindcastDates.append(hindcastDate)
           
            #Please note: the steps 4 and 5 below could run in parallel
           
            #Step 4: Get all the available perturbed forecast, pressure level data
            pfplTarget = "%s_%s_%04d%02d.grb" % (
                origin, "pfpl", hindcastYear, hindcastMonth)
            BoM_reforecast_pf_pl_request("/".join(hindcastDates), pfplTarget)
           
            #Step 5: Get all the available perturbed forecast, surface data
            pfsfcTarget = "%s_%s_%04d%02d.grb" % (
                origin, "pfsfc", hindcastYear, hindcastMonth)
            BoM_reforecast_pf_sfc_request("/".join(hindcastDates), pfsfcTarget)

def BoM_reforecast_pf_pl_request(hindcastDate, target):
    """
       A BoM reforecast, perturbed forecast, pressure level, request.
       The cost of this request is 571,392 fields and 11.1352 Gbytes 
       Change the keywords below to adapt it to your needs.
    """
    server.retrieve({
        "class": "s2",
        "dataset": "s2s",
        "date": modelVersionDate,
        "expver": "prod",
        "hdate": hindcastDate,
        "levtype": "pl",
        "levelist": "10/50/100/200/300/500/700/850/925/1000",
        "origin": origin,
        "param": "130/131/132/133/135/156",
        "step": "24/to/1488/by/24",
        "stream": "enfh",
        "target": target,
        "time": "00",
        "number": "1/to/32",
        "type": "pf",
    })

def BoM_reforecast_pf_sfc_request(hindcastDate, target):
    """
       A BoM reforecast, perturbed forecast, sfc request.
       The cost of this request is 383,040 fields and 7.1 GB
       Change the keywords below to adapt it to your needs.
    """
    server.retrieve({
        "class": "s2",
        "dataset": "s2s",
        "date": modelVersionDate,
        "expver": "prod",
        "hdate": hindcastDate,
        "levtype": "sfc",
        "origin": origin,
        "param": "31/34/121/122/136/146/147/151/167/168/169/175/176/177/179/180/181/235/228086/228095/228096/228141/228143/228144/228164/228228",
        "step": "24/to/1488/by/24",
        "stream": "enfh",
        "target": target,
        "time": "00",
        "number": "1/to/32",
        "type": "pf",
    })

if __name__ == '__main__':
    retrieve_BoM_reforecast()
                                          

• BoM re-forecast requests
• ECMWF Web API Home
• BoM re-forecast availability
• A brief description of reforecasts