Soak the filters flat rather than replacing them, since heritage St George filters are unlikely to be obtainable. The more significant question on a hood this old is the ducting above it, which has had decades of cooking vapour through it.
A St George rangehood in an older Sydney home has usually been running since the kitchen was built, which frequently means the 1980s or 1990s.
Thirty five years of duct
This is the finding that matters on a hood of this vintage, and it is not about the appliance.
Fat condenses on the inside of a duct and hardens, layer over layer, for as long as the hood runs. A duct fitted in the early 1990s has had thirty five years of that, and nobody has ever seen it, because it disappears into a ceiling or a wall cavity and there is no reason to look.
Two consequences. The duct is progressively restricted, so the hood moves less air every year in a decline slow enough that nobody notices. And a duct heavily lined with hardened fat is a genuine fire risk, which is why commercial kitchens have theirs cleaned on a schedule and domestic ones essentially never do.
On a St George hood the ducting is more likely to be the real problem than the filters are, and it is worth establishing before paying anyone to clean the visible parts.
Filters you probably cannot replace
Heritage St George filters are unlikely to be available, which inverts the usual advice. Rather than soaking once and replacing if it fails, soak twice with a change of water and give it considerably more patience than you would give a filter you could renew.
Where a filter is genuinely finished, measure it. Standard rectangular mesh sizes are more interchangeable than model-specific ones, and there is a fair chance yours is a common dimension in a St George frame.
The general method is on our rangehood cleaning guide.
The impeller behind the filters
With the filters out, look up with a torch. The impeller sits in a housing above, and on a hood that has run for three decades that housing carries a hard brown lacquer of everything the filters ever missed.
That deposit is behind all three complaints people have about old hoods. The smell is warm grease in a warm housing. The noise is a fan thrown out of balance by uneven build-up. The poor extraction is a loaded impeller moving less air.
Isolate the power at the wall or the circuit breaker before anything goes near it. On an installation of this vintage the switch is frequently not where you would expect and occasionally not labelled at all, so establish it properly rather than assuming.
What actually goes wrong
Decades of ducting nobody has inspected.
A filter discarded that could not be replaced.
An impeller housing never opened, which is where the smell and the noise come from.
Should you do it yourself?
Do it yourself for the filters, patiently and regularly.
Get someone in to establish what the duct actually looks like, which on a hood this old is the question worth answering. The St George oven and St George cooktop are usually the same age.