Staff Engineer - Backend

The Onset; Sydney; On-site; AUD 180,000 - 280,000; Full time

Job summary

The Onset is building a greenfield payments and billing platform in Sydney. You’ll join a small founding team as a Staff/Principal Backend Engineer, shaping core architecture and API design.

You’ll own how services interact, ensure reliability, and drive decisions in a fast-moving, autonomous environment. Lead backend work across payments, pricing, and APIs, with a strong emphasis on correctness and scalability in a cloud-native setting.

Qualifications

• Strong Java/JVM experience with distributed backend systems.
• Experience designing and building payment/billing infrastructures.
• Ability to own complex technical decisions end-to-end.

Responsibilities

• Design and own backend components for payments & billing platform.
• Define service boundaries and API designs for customers building on the platform.
• Ensure reliability, performance, and observability of core infrastructure.
• Collaborate with founders to shape architecture and engineering standards.

Skills

Java

JVM

Distributed systems

APIs

Cloud infrastructure

Backend architecture

Ownership

Tools

Spring

Kubernetes

Job description

Staff / Principal Backend Engineer – Payments Infrastructure

Sydney | Early-stage | Greenfield | Java / JVM | Payments & Billing

This is an opportunity to join a well-backed software company at the point where the core engineering foundations are still being built.

The business is building payments and billing infrastructure for software companies. The problem goes well beyond processing a payment. It covers how SaaS businesses price products, manage billing, process transactions, expose APIs and eventually automate much of the operational work that currently sits around revenue.

Very little has been decided for you.

You’ll join a small founding engineering team and have a major say in how the platform is designed, how the systems fit together and what the engineering standards look like as the company grows.

What you’ll be building The backend will sit at the centre of a complex payments and billing platform.

You could be working across areas such as:

• payment and billing infrastructure
• pricing and subscription logic
• APIs and integrations
• transaction and event processing
• distributed systems and service architecture
• reliability, performance and observability
• data flows across payments, customers and products
• systems that can support increasingly automated billing and pricing decisions

This is genuine greenfield engineering. You won’t be inheriting years of legacy architecture or joining to maintain somebody else’s decisions.

You’ll help make those decisions.

The role This is a senior individual contributor position for someone comfortable operating with a high degree of autonomy.

There won’t be a detailed technical roadmap handed to you with every decision already made. You’ll work closely with the founders and other senior engineers to understand the problem, work through the trade-offs and decide how things should be built.

That means getting involved in questions such as:

• Where should service boundaries sit?
• What belongs in the core domain model?
• How should APIs be designed for customers building on top of the platform?
• What needs to be synchronous versus event-driven?
• How do we design for correctness when money is involved?
• Where do we need strong consistency and where can we accept eventual consistency?
• How do we build something simple today without creating problems at scale later?

You’ll still write plenty of code, but the value of this role is not measured by ticket throughput.

The expectation is that you can take an unclear engineering problem, work out what matters, make sound technical decisions and get it into production without needing somebody to constantly point you in the right direction.

What we're looking for You’re likely already operating at Staff, Principal, Lead or senior Founding Engineer level, even if your current title says something different.

We’re particularly interested in engineers who have:

• designed and built complex backend systems
• made architecture decisions that affected more than one service or team
• strong Java and JVM experience
• worked with distributed systems, APIs and modern cloud infrastructure
• built systems where reliability and correctness genuinely mattered
• operated well when requirements were incomplete or changing
• taken ownership from problem definition through to production
• influenced technical direction without needing formal management authority

Experience building payments, billing, financial infrastructure, ledgers, transaction systems or similarly complex platforms would be particularly relevant.

People from strong SaaS, fintech, infrastructure or platform engineering backgrounds could also fit well if they have dealt with comparable systems problems.

Why consider it? The main reason is the stage of the company.

You’re early enough that your decisions will genuinely shape what gets built.

The architecture isn’t settled. The engineering culture isn’t inherited. There isn’t a large organisation between you and the problem.

You’ll be working directly with experienced founders and a small group of senior engineers on a product where the backend is the product.

For someone who has spent the last few years operating inside established systems and wants to build something from the foundations again, this is a rare opportunity to do it with real scope and responsibility.

It will suit someone who wants ownership rather than oversight, and who is comfortable making decisions when there isn’t an existing playbook to follow.

If you’ve built important backend systems before and want your next role to put you much closer to the architecture, product and company decisions, I’d be interested in speaking.