I have been a 14-16 (sleep deprived after DD2) and now am 8-10 after 5:2 diet and exercise. DS was a toddler and waking up at 5am so I used to pop him in front of CBeebies on the iPad and do a Jillian Michaels exercise DVD. Meant I started the day feeling good rather than knackered!
I live in a fairly well off area, not the sort of place where people use nannies for longer than working hours, nor do people have housekeepers, chefs or night nannies, but plenty of people have cleaners and belong to gyms. People here do plenty of exercise (there's a big mum's running community with free social runs), and family activities tend to be active walks, bike rides etc, children have trampolines so burn off energy like that. I definitely notice that children here are slimmer than in the less well off areas, as are the parents. Being slim, active and eating healthily are normalised here. Most of my friends are Aldi shoppers but the trolleys have plenty of fruit and veg in and people generally cook from scratch, don't buy fizzy drinks etc. DH grew up working class and thinks my obsession with cooking from scratch, rather than using freezer food, and eating veg, rather than crisps and biscuits, is some sort of strange affectation!
I think one key to staying slimmer is no snacks. When you graze all day it's so easy to take in loads of extra calories without really noticing it. Plus your body stays in fat storage mode rather than fat burning mode. If you eat decent sized meals with plenty of lean protein, plenty of fibre and some good fats you can easily last until your next meal. High carb/processed junk will just leave you craving more of the same. I notice a definite difference in my mood when I eat well vs when I have processed food.
In the summer a typical lunch might be a slice of toasted rye bread with smoked mackerel pate and a salad with olive oil dressing. Winter lunches are homemade veg and lentil or bean soups, or omelette with steamed veg. Breakfast is yogurt with a sprinkle of homemade low sugar granola and some berries, or boiled eggs, tomatoes and spinach. Dinner is normal family food like chilli con carne, pasta bolognese, curry etc but I have a small portion of carbs and add extra salad/veg.
Also- don't drink your calories! Frothy coffees, juices, smoothies and alcohol add lots of extra calories into your day. I normally only drink alcohol on the weekends and holidays.
You mentioned you are in Oz OP, one of the fattest nations on earth statistically. I think that would surprise most brits as we think of Aussies as super sporty and healthy. When we lived there I put on a lot of weight initially as the portion sizes are giant and people eat out a lot more than in the UK. Depending on where you are there can be more of an American culture of fast food and driving everywhere too. Plus Australians are obsessed with juice bars and smoothies, and they give lots of calories with no fibre. I found fruit and veg quite pricey and the supermarkets were uninspiring. But if you can shop in local veg shops and fishmongers there are good deals to be had.
Agree that you have a DH problem- he needs to prioritise the health of both of you, not just him. Share evenings at the gym, if he needs to go every day he can alternate going on mornings, or go out again after you have been. Or if you have no interest in the gym he needs to share evenings putting the DC to bed so you can work/rest/meditate/whatever.