These words were used in a radio Mother’s Day ad for Outback Steakhouse. The ad originated in the US—after all, Outback Steakhouse is an American company, founded in 1988—but they’re obviously trying to sound Australian. No date is given for the ad, but comments indicate it is old, and it used to be used every year before (American) Mother’s Day but has been discontinued. I assume all the commenters are American, and they seemed to know the song only from the ad.

Thanks to GUEST,999 for providing the link. This is my transcription:

No one ever loves you like your mum, mum.
Ev’rything we owe, we owe to mum (we owe to mum), mum.
Even when she whacks you on the bum (whacks your bum), mum,
Mum’s the word and so the word is mum (that’s me mum!), mum.

One mother’s day a year is fine for some, mum,
But we know our mother’s job is never done (it’s never done), mum.
You will always be our number one, mum.
Mum’s the word and so the word is mum (that’s me mum!), mum.